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Resource recovery and circular economy from organic solid waste using aerobic and anaerobic digestion technologies
Journal Pre-proofs Review Resource Recovery and Circular Economy from Organic Solid Waste using Aerobic and Anaerobic Digestion Technologies Steven Wainaina, Mukesh Kumar Awasthi, Surendra Sarsaiya, Hongyu Chen, Ekta Singh, Aman Kumar, B. Ravindran, Sanjeev Kumar Awasthi, Tao Liu, Yumin Duan, Sunil Kumar, Zengqiang Zhang, Mohammad J. Taherzadeh PII: DOI: Reference:
S0960-8524(20)30047-X https://doi.org/10.1016/j.biortech.2020.122778 BITE 122778
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
18 November 2019 6 January 2020 7 January 2020
Please cite this article as: Wainaina, S., Kumar Awasthi, M., Sarsaiya, S., Chen, H., Singh, E., Kumar, A., Ravindran, B., Kumar Awasthi, S., Liu, T., Duan, Y., Kumar, S., Zhang, Z., Taherzadeh, M.J., Resource Recovery and Circular Economy from Organic Solid Waste using Aerobic and Anaerobic Digestion Technologies, Bioresource Technology (2020), doi: https://doi.org/10.1016/j.biortech.2020.122778
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Resource Recovery and Circular Economy from Organic Solid Waste using
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Aerobic and Anaerobic Digestion Technologies
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Steven Wainainab, Mukesh Kumar Awasthia*,b, Surendra Sarsaiyaf, Hongyu Chend, Ekta
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Singhc, Aman Kumarc, B. Ravindrane, Sanjeev Kumar Awasthia, Tao Liua, Yumin Duana,
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Sunil Kumarc, Zengqiang Zhanga, Mohammad J. Taherzadehb
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a
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Shaanxi Province 712100, PR China
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bSwedish
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cCSIR-National
College of Natural Resources and Environment, Northwest A&F University, Yangling,
Centre for Resource Recovery, University of Borås, 50190 Borås, Sweden Environmental Engineering Research Institute, Nehru Marg, Nagpur -
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440 020, Maharashtra, India
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dInstitute
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eDepartment
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Gu, Suwon, Gyeonggi-Do, 16227, South Korea
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fKey
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Ethnomedicine of Ministry of Education, Zunyi Medical University, Zunyi, Guizhou,
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China
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*Corresponding author:
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Dr. Mukesh Kumar Awasthi
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E-mail: [email protected] (M.K. Awasthi)
of Biology, Freie Universität Berlin Altensteinstr. 6, 14195 Berlin, Germany of Environmental Energy and Engineering, Kyonggi University Youngtong-
Laboratory of Basic Pharmacology and Joint International Research Laboratory of
20 21
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Resource Recovery and Circular Economy from Organic Solid Waste using
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Aerobic and Anaerobic Digestion Technologies
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Steven Wainainab, Mukesh Kumar Awasthia*, b, Surendra Sarsaiyaf, Hongyu Chend, Ekta
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Singhc, Aman Kumarc, B. Ravindrane, Sanjeev Kumar Awasthia, Tao Liua, Yumin Duana,
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Sunil Kumarc, Zengqiang Zhanga, Mohammad J. Taherzadehb
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Abstract
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With the inevitable rise in human population, resource recovery from waste stream is
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becoming important for a sustainable economy, conservation of the ecosystem as well as
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for reducing the dependence on the finite natural resources. In this regard, a bio-based
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circular economy considers organic wastes and residues as potential resources that can be
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utilized to supply chemicals, nutrients, and fuels needed by mankind. This review
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explored the role of aerobic and anaerobic digestion technologies for the advancement of
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a bio-based circular society. The developed routes within the anaerobic digestion domain,
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such as the production of biogas and other high-value chemicals (volatile fatty acids) were
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discussed. The potential to recover important nutrients, such as nitrogen through
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composting, was also addressed. An emphasis was made on the innovative models for
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improved economics and process performance, which include co-digestion of various
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organic solid wastes, recovery of multiple bio-products, and integrated bioprocesses.
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Keywords: Resource recovery; organic solid waste; circular economy; anaerobic
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digestion; composting.
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1. Introduction
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Disposal of organic solid waste (OSW) has become one of the hot topics in research.
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Simultaneously, the collection and recycling of OSW is presently principal environmental
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concerns in both urban and countryside areas in numerous industrial and developing
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countries (Vinay et al., 2018). With the rapid development of modern society and the
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continuously rising population all over the world, the generation of OSW is projected to
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be significantly increased annually. Statistically, 2.01 billion tons of municipal wastes
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were produced worldwide and by 2050, approximately 3.4 billion tons of municipal
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wastes will be emerged annually if the current trend continues (Luis et al., 2019).
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Furthermore, improper waste management is not only pernicious to human beings and
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local environment, but also exacerbates the climate change and causes the sanitation
54
system thus forcing nations and governments to invest more financial and material
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resources for its remediation. Consequently, ideal technologies to OSW management is
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not only conducive to environmental protection and sustainable development but also
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critical to forge a circular economy (Felix et al., 2019).
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In a circular economy, recycling and reuse is the major principle for designing and
59
optimization of products. As nations and authorities all over the world gradually
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embracing the concept of circular economy and attempting to bring it in practice,
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disposing wastes with an advanced and sustainable way could promote efficient economic
62
growth with reduced environmental impacts at the same time (Pooja et al., 2018;
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González-García et al., 2019). This concept not only creates more job opportunities but
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also able to converts waste into treasure and transforms it to a new clean energy and
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material sources through classified management and recycling. Governments and
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municipalities around the world have started paying great attention to it and constantly
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reform the urban wastes management system, making it an important part of the green
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economy and sustainable development (Biswabandhu et al., 2019).The majorities of OSW
69
are composed of animal wastes, municipal and agricultural wastes, thus OSWs are
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generally rich in nutrient substances, such as proteins, minerals, and sugars, which can be
3
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reused as raw materials in the production of green energy (Kiros et al., 2017). Nevertheless,
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due to the restrictions of the allocated budgets, infrastructure development, OSW
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management is still suffering from improper treatment in most developing and even in
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few developed countries. Therefore, the most decisive factors in OSW management is to
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develop an economically sustainable process followed by technically feasible, socially
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and legally acceptable as well as environment friendly. Hence, OSW management is
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usually one of the most important challenges for the authorities and a hot research topic
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all over the world (Alejandra et al., 2018). Based on the characteristics of the OSW, they
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are generally treated to be converted into organic fertilizer or directly transport to landfill
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sites or incineration facilities. Therefore, the conventional methods including landfill,
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incineration, and composting are traditional matured technologies for OSW management.
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Different processing technologies possess varied advantages and also drawbacks as well.
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On the other hand, biological technologies for OSW treatment are now-a-days seriously
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considered in the world because of the characteristics of low energy consumption, low
85
cost and investment, high organic removal rate and meeting the requirement of circular
86
economy (Nurul et al., 2018). However, due to the different economical possibilities of
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various countries and the OSW treatment technologies, traditionally, the most commonly
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applied technologies are still landfill in developing countries and incineration in most of
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the developed countries.
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Composting and anaerobic digestion (AD) depend on the decomposition and
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degradation of organic matter in OSW to stable substances, such as humus and digestate
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could be applied to soil as a fertilizer or soil improvement if their qualities are at standard
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level. During the AD process, biogas that is rich in methane and carbon dioxide are
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produced, which could be used as a fuel for combustion in transport or energy production
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(Nuhaa, 2019). Meanwhile, though these two technologies are not new, they probably
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could have a key role in achieving circular economy objectives to divert OSW from
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landfill and incineration and improving the circularity of biological nutrients. However,
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there are still many practical and technical issues that need to be addressed in the
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application of these two biological treatment methods (Zhang et al., 2019). In Europe, 60
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million tons of OSW could be potentially recycled by AD and composting technologies
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(Felix et al., 2019). This would save about one million tons of nitrogen and 20 million
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tons of organic carbon (Luis et al., 2019). At present, most of these nutrients are lost
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through landfilling organic waste. The European Commission reported that the European
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countries in average recycle only 5% of the total OSW. It is estimated that if a higher
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portion of OSWs could be recycled and reused, it could replace approximately 30% of the
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chemical fertilizer applied to soil, that means 1.8 million tons phosphate fertilizer every
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year (Luis et al., 2019).
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OSW can be converted to vary products through different processing technologies,
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such as fertilizer, heat, clean fuel from AD, plant and soil food from composting as well
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as methane from landfill. Despite this, either composting or AD must be fully understood
111
prior to their wider deployment. This review comprehensively describes the current
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generation state of the OSW and different disposal technologies, particularly for
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composting and AD and elaborating on the details of their treatment process and their
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environmental, social and economic applicability. Both of these two technologies are not
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completely economically feasible, because of which they have not been successful applied
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in many countries. A deep insight has been attempted in this paper to make a better
117
understanding for the importance of its economic viability.
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2. Organic solid waste generation
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The constant increase of worldwide population to more than seven billion plus the
120
continuous improvement of global welfare has resulted in a rapid increase of the daily
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consumption of resources and products, thus tremendous amount of recyclable wastes is
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generated (Alejandra et al., 2018). The generation of wastes per capita in most of the
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industrial countries are higher than developing countries, which is most likely connected
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to the socio-economic status of the country. Taking the EU as an example, EU-28 nations
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produced nearly 25.38 million tons of wastes through all the activities including economic
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and households in 2016. Amongst it, the specific waste output of each EU nation also
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illustrated that the total amount of wastes generated had a certain degree of negligible
128
relationship with the economic scale and population of a nation (Forough et al., 2019). It
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can be clearly observed from Table 1 that the EU member states with the smallest
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population and area possessed the lowest waste generation, oppositely the large nations
131
owned the highest production. However, Italy had a relatively high amount of wastes
132
generation, while Bulgaria and Romania possessed relatively low level of waste.
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It is reported that in urban and rural areas of EU and other Asia countries, the level
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of solid wastes generation ranges from 0.9-1.6, and 0.7-1.5 kg per capita per day,
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respectively while the urban areas generate approximately 1.3 billion tons of solid wastes
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in 2015. It is estimated that it will increase to 2.2 billion tons by 2025 (WBA, 2017).
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Generally, the produced solid wastes are composed by agricultural and industrial wastes
138
including leather, wood, glass, paper, green waste biomass, household wastes, and
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electronic wastes, such as computers, telephones, refrigerators, and televisions as well as
140
also construction, demolition wastes and medical wastes (Munawar et al., 2019). As
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mentioned above, in most of the developing countries, the massive population growth and
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the acceleration of urbanization as well as the continuous improvement of people’s living
143
standards have significantly resulted in increasing the amount of urban wastes. The
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population all over the world was about 7.2 billion in 2013 and it will reach to 9.6 billion
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by 2050 (Biswabandhu et al., 2019). The population in urban areas is prognosed to account
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for 86% and 64% of the total population in developed and developing countries,
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respectively (Silva et al., 2019). Meanwhile, the dramatic increase of urban population
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will also result in infrastructural and strategic challenges as well as adverse land-use
149
changes. Waste to energy technologies could turn as wise choices to deal with these
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circumstances and provide long-term and cost-effective solutions for the growing energy
151
requirements and sustainable waste management. It is estimated that only from the global
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urban waste collection market can produce around $410 billion income (Waste
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Management World, 2017). However currently, only a quarter of wastes are recycled.
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Hence, recycling and utilizing OSW by applying biotechnologies has great bright
155
prospects and market in the near future (Kiros et al., 2017).
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3. Current Organic Solid Waste Treatment Technologies
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Various options are available for converting organic solid wastes into useful
158
resources. Although pure cultures are ideal or synthesizing chemicals at high specificity
159
and thus requiring minimal purification prior to final use, they require single substrates in
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the form of e.g. sugars. This has major drawbacks as it raises the cost of production and
161
also competes for resources required for growing food and feed for humans and animals
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raising ethical debates. Natural mixed bacterial consortia are therefore an attractive option
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for treating the abundantly available heterogeneous solid wastes and enables resource
164
recovery in the form of energy and high-value platform chemicals through anaerobic
165
digestion. These bacterial systems are also robust and have the capacity to withstand
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stressful micro-environments. Among the biological solid waste treatment possibilities
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provided in this section, only the production of biogas has been widely commercialized.
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The other products of ADs are mainly at research stage, providing opportunities for further
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scientific investigations and development (Wainaina et al., 2019a).
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3.1. Biogas production
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AD is an established biological processing technique suitable for stabilizing a
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plethora of organic solid wastes coupled with recovery of energy and nutrients. It provides
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a sustainable option for providing renewable materials and fuels thereby contributing to
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circular economy (Antoniou et al., 2019). Indeed, there are indications of that the biogas
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industry is steadily growing due to apparent substantial reduction of the capital and
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operating costs of AD facilities. Indeed, it has been estimated that the cost of production
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of biogas in AD facilities would reduce by about 38% in 2050 compared to 2015 (Ecofys,
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2018). Globally, it the estimated that biogas production had an average growth rate of
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11.2% reaching up to 58.7 billion Nm3 in 2017 (WBA, 2017).
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The AD process takes place in oxygen-deficient environments and is catalyzed by
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natural microbial communities in a four-stage complex process resulting into mainly
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biogas. A pretreatment step, carried out either biologically, chemically or physically, can
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precede the actual anaerobic digestion process in order to adequately prepare substrates
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with difficult structures such as lignocelluloses (Taherzadeh and Karimi, 2008; Sarsaiya
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et al., 2019a). The AD process kinetics are determined by the nature of substrates and the
186
physico-chemical parameters, such as the pH, temperature and hydraulic retention time
187
(Weiland, 2010). The details of the reactions taking place during the breakdown of the
188
solid wastes during AD are provided in previous reviews (Aryal et al., 2018; Merlin
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Christy et al., 2014; Weiland, 2010). In summary, in the first stage hydrolytic microbes
190
break down the complex macromolecules into simple monomers with the help of enzymes
191
such as amylases, lipases, and proteases. The second stage applies acidogenic microbes
192
and converts the soluble products produced in the first step into biomolecules, such as
193
alcohols, volatile fatty acids (VFAs), hydrogen (H2) and carbon dioxide (CO2). These
194
products then serve as substrates to acidogens for synthesis of mainly acetic acid in the
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third stage. The final stage is the most sensitive and greatly relies on the success of the
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previous reactions in which methanogens convert acetic acid, hydrogen and carbon
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dioxide into biogas. The methanogens follow mainly the acetotrophic (aceticlastic) and
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Wood Ljungdahl pathways for production of CH4 from acetic acid and the reaction
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between H2 and CO2, respectively, with the latter being more common (Aryal et al., 2018;
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Merlin Christy et al., 2014; Rusmanis et al., 2019).
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The chemical composition of raw biogas from a typical solid waste AD facility is
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based on the process conditions. It consists of 50 – 75% CH4, 30 – 50% CO2, 0 – 3% N2,
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~ 6% H20, 0 – 1% O2, 72 – 7200 ppm H2S, 72 – 144 ppm NH3 and other minor impurities
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(Kapoor et al., 2019; Yentekakis and Goula, 2017). The raw biogas can be directly applied
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for generating electricity and heat while upgraded gas (biomethane) can be injected into
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the natural gas grid or used as vehicle fuel (Figure 1). The latter application has been
207
reported to having the potential of aiding in reducing road transport - related greenhouse
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gas (GHG) emissions by up to 25% (Olsson and Fallde, 2015). In order to use the biogas
209
as a transport fuel, an extra treatment is required for removal of CO2 and other impurities,
210
such as H2S and water vapor. Gas cleaning is the initial step for removal of impurities that
211
could damage mechanical and electrical appliances during the use of biogas and can be
212
achieved using adsorption with silica gel and activated carbon or molecular sieves.
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Upgrading techniques mainly for separation of CO2 from CH4 in order to raise the calorific
214
value of the gas are water scrubbing, pressure swing adsorption, cryogenic technology,
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membrane separation and organic scrubbing using amines such as diethanolamine,
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diglycolamine and monoethanolamine (Kapoor et al., 2019).
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3.2. Hydrogen production
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Energy recovery from solid wastes can also be achieved via the dark fermentation
219
process. Compared to the conventional production routes such as steam reforming of
220
methane and non-catalytic partial oxidation of fossil fuels, production of hydrogen from
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wastes not only reduces the reliance on the depleting petroleum resources but at the same
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time it is less energy-intensive as well (Kapdan and Kargi, 2006). Hydrogen has a higher
223
heating value compared to methane (120 vs. 50 MJ/kg) (Liu et al., 2013). It’s thus a
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potential clean energy source especially for the transport industry. Its other areas of
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application include production of electronics, processing of steel as well as hydrogenation
226
of fats and oils (Kapdan and Kargi, 2006). A number of organic solid wastes that have
227
been explored for hydrogen production are provided in Table 2.
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The dark fermentation process is principally an intermediate within the AD
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process in which hydrogen is released. Thus, relevant parameters suitable for biosynthesis
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of hydrogen while at the same time deactivating the methanogens that would otherwise
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consume it are optimized (Kapdan and Kargi, 2006). Some of the important process
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parameters for ideal hydrogen production include the pH level, temperature and bioreactor
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design. Fang and Liu (2002) reported an optimal pH level of 5.5 while mesophilic and
234
thermophilic temperature ranges were found to stimulate hydrogen production from
235
wastes compared to psychrophilic conditions (Wang and Wan, 2008; Wang and Wan,
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2009). Hydrogen fermentation systems also require appropriate bioreactor design to
237
supply necessary hydrodynamic properties such as sufficient mixing (Ren et al., 2011). It
238
is worth mentioning that the presence of other by-products, such as volatile fatty acids
239
(VFAs) at certain concentrations can negatively impact on the microbial hydrogen
240
production due to potential inhibition or even the depletion of hydrogen associated with
241
formation of some VFAs. Potential solutions, such as the use of membrane bioreactors,
242
have been proposed as effective in removing the VFAs as they are being formed and
243
thereby improving the gas production (Trad et al., 2015).
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3.3. Production of soluble biochemicals
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3.3.1. Volatile fatty acids
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VFAs are short-chain aliphatic carboxylic acids also formed as intermediaries
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during the AD process before the methanogenesis stage and consist between 2 – 6 carbon
248
atoms. Petroleum resources are currently utilized for industrial synthesis of VFAs and
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replacing these resources with solid wastes as the primary feedstock creates and attractive
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circular economy pathway. The success of the acidification in AD reactors depends on
251
optimization of operating parameters, such the pH, temperature, organic loading rate
252
(OLR) and the hydraulic retention time (HRT) as well as employing appropriate means to
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deactivate the methanogenic VFAs-consumers (Atasoy et al., 2018; Lim et al., 2008,
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Wainaina et al., 2019a). Table 3 gives the performance of some waste-to-VFAs systems.
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These acids have the potential for use as building blocks in a wide spectrum of
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industrial processes. For instance, acetic acid can be applied in pharmaceutical industries,
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propionic acid for manufacture of paints, butyric acid for manufacture of perfumes while
258
caproic acid is used for preparation of food additives. The mixed solution of these VFAs
259
also has important applications in wastewater treatment plants for biological removal of
260
nitrogen and phosphorus, biosynthesis of mixed alcohols as well as production of
261
biodegradable plastics. Moreover, the VFAs can be applied to generate electricity in
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262
microbial fuels cells in addition to serving as the carbon source for bioprocessing of
263
biodiesel (González-García et al., 2019).
264
In order to facilitate practical production systems, recovery and purification of
265
VFAs from fermentation broths are crucial. This involves primary separation of the liquid
266
VFA fraction from the solid contents in the bioreactor made up of the cells and nutrients,
267
which might be sufficient for some applications. However, for some application areas,
268
more complex fractionation is needed for attaining the strict purity requirements.
269
Adsorption, gas stripping and electrodialysis are examples of relevant methods proved to
270
be effective in separation of the VFAs in model carboxylate solutions (Atasoy et al., 2018).
271
Some researchers have also designed systems with the recovery procedure running
272
concurrently with the actual microbial process in the so-called in situ product recovery
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using membrane-aided techniques (Arslan et al., 2017; Wainaina et al., 2019b). The
274
benefits of such schemes are the supply of the carboxylates for subsequent processing
275
without introduction of foreign soluble chemicals, in addition to the relief on the microbes
276
from potential harmful effects of un-dissociated acid molecules.
277
3.3.2. Lactic acid
278
Lactic acid is a carboxylate with a market potential of USD 3.82 billion by 2020
279
used for production of acrylic acid, biodegradable polymers, pyruvic acid, 1, 2-
280
propanediol, among others (Bonk et al., 2017; Phanthumchinda et al., 2018). It’s main
281
area of application is as a building block for poly (lactic acid) which has a high estimated
282
market value of USD 5.16 billion by 2020 due to the mounting ecological concerns and
283
government policies (Phanthumchinda et al., 2018). A significant portion of the global
284
industrial production is from fermentation, which is controlled by factors such as pH,
285
temperature and nitrogen concentration (Alves de Oliveira et al., 2018).
286
The lactic acid bacteria (LAB) are popular for processing single sugars into lactic
287
acid. In the AD systems, a variety of organic residues, such as food wastes and organic
288
fraction of municipal solid waste, can be processed using microbes such as Lactobacilli
11
289
during the acidogenic phase (Gu et al., 2018; Probst et al., 2015). Gu et al. (2018) for
290
instance achieved a lactic acid buildup of up to 74.3% in a leach bed reactor treating food
291
waste. Another research by Bonk et al. (2017) developed an AD reactor coupled with a
292
percolation system which yielded lactic acid at 0.15 g chemical oxygen demand equivalent
293
per g total solids fed (gCOD/gTSfed) from food waste with lactic acid concentration of up
294
to 16.4 g COD/L. As with the VFAs, appropriate product separation and purification
295
methods are required to enhance the purity. One of the possible efforts involved a
296
membrane-assisted lactic acid recovery method that incorporated microfiltration for cell
297
removal, ultrafiltration for protein removal, and reverse osmosis for concentrating the
298
lactic acid at high purity (Phanthumchinda et al., 2018).
299
3.3.3. Other potential biochemicals
300
The AD process also synthesizes other useful metabolites, although in low
301
concentrations. According to Zhou et al. (2018), the possible metabolic pathways during
302
the acidogenic biodegradation of wastes can lead to biosynthesis of other chemicals
303
alongside VFAs, lactic acid and hydrogen. One such metabolite is succinic acid which can
304
be applied in key industrial processes such as manufacture of inks, polymers and
305
pharmaceuticals, and is bio-synthesized via the tricarboxylic acid cycle (TCA) (Isar et al.,
306
2006). The use of pure microbes such as Actinobacillus succinogenes and an engineered
307
Escherichia coli strain was found to effectively convert food waste hydrolysate into
308
succinic acid (Sun et al., 2014). Concentrations of up to 8.5 g/L were detected during
309
biodegradation of food waste at pH 5.0 (Lim et al., 2008). Such a performance indicates
310
the potential for mixed bacterial consortium for metabolizing succinate that could be
311
investigated in future studies. Ethanol has also been detected in small amounts from
312
anaerobic digestion digesters. For instance, acidification of food waste yielded low
313
concentrations at pH 6 and 7 (Xiong et al., 2019). In another study, Zhang et al. (2015)
314
found that ethanol made up 33.8% of the liquid fraction during the thermophilic AD of
315
glucose. One proposal to raise the concentration of ethanol by Zhou et al. (2018) would
316
be providing high H2 partial pressure in the head space of acidification reactors.
12
317
3.4. Nutrient recovery
318
The anaerobic digestion (AD) is considered as one of the foremost cost-effective
319
biological treatments of the biowaste. It permits vitality recuperation and the generation
320
of nutrient-rich digestate whereas lessening natural impacts of the waste transfer. It
321
contains most of the supplements from the input material and can be utilized as fertilizer
322
or organic amendment for farming. The natural supplement circle locally the digestate that
323
needs to be utilized in ranches found adjacent to the mAD (micro-anaerobic digestion)
324
units (Thiriet et al., 2020). The biowaste is utilized to provide vitality and recover other
325
nutrients not only because it advances towards the financial conditions of a country, but
326
besides makes a difference towards financial change of human society (Bhatia et al., 2018).
327
Recognizably, the two forms, i.e., anaerobic digestion and composting, facilitates together
328
since the solid fraction of digestate can be treated vigorously so as to recover both
329
renewable energy and nutrients from common waste (Micolucci et al., 2016). These
330
profitable nutrients display in natural strong waste which are efficiently misplaced, can be
331
recovered through appropriate biochemical technologies and utilized as nutrient-rich
332
fertilizers in rural areas for keeping up soil richness and arrive rebuilding hones at low-
333
input premise. In this way, impressive emphasis is being focused on receiving biologically
334
sound on best of financially maintainable and socially acceptable tools which reinforce
335
potential recuperation of nutrients, with least contamination load (Soobhany, 2019). The
336
final product, compost, may be a result of microbial action and contains mineral nutrients
337
such as nitrogen, phosphorus and potassium (Figure 6). Unfortunately, for nutrients
338
recuperation it is exceptionally weak material in its raw form, but can contribute in 9.4%
339
of P and 5% of N presented with inorganic fertilizers individually (Chojnacka et al., 2019).
340
TeraxTM technology gives numerous opportunities for resource recovery such as
341
changing over waste into value-added items, nutrient recuperation from natural waste, and
342
metal expulsion from natural waste (Munir et al., 2018). There are still various challenge's
343
heads for AnMBR stage applications to be handled, especially for extreme film fouling,
344
moo methane substance in biogas, exceedingly broken-down methane, destitute smelling
13
345
salts expulsion and phosphorus recuperation, etc. To address the above issues, a new-
346
generation process (Figure 6), i.e. so-called “Integrated Multistage Bio-Process (IMBP)”
347
constituted of solar-driven bioelectrochemical framework (BES)-AnMBR, fractional
348
nitritation/anammox (PN/A), nitrate decrease through anaerobic oxidation of methane
349
(AOM) and biological/chemical phosphorus precipitation units have been examined in
350
this article, with flexible capabilities in synchronous biowastes valorization, CO2 electro-
351
methanogenesis and synchronous biogas overhauling, in-situ fouling control, alkali
352
expulsion, broken up methane reutilization, and phosphorus recoup as hydroxyapatite-rich
353
supplements (Zhen et al., 2019).
354
3.5. Composting/co-composting
355
Composting process occurs naturally in the environment, although efficient
356
composting is needed to control several factors to avoid environmental challenges such as
357
odor and dust, and for obtaining a quality agricultural manure. Generally, composting
358
process are divided into two phases, the bio-oxidative phase and the maturing phase also
359
named as curing phase (Bernal et al., 1996). The bio-oxidative phase again carried out in
360
three stages, such as initial activation, thermophilic and mesophilic or maturation phase.
361
The organic waste degradation occurs during the thermophilic phase and microorganisms
362
degrade the available compounds in the organic waste (Keener et al., 2000). In this phase,
363
high temperature generation in the composting pile observed due to the heat generated
364
from the composting pile due to the microbial degradation of organic waste (Sarsaiya et
365
al., 2018). Generally, high temperature is required i.e. 55 oC to destroy pathogens in the
366
organic solid waste and usually composting piles reach temperatures as high as 70 oC
367
during the degradation of animal manure and green waste (Singh and Kalamdhad, 2014;
368
Tang et al., 2011; Cáceres et al., 2015). In addition, in this stage the organic compounds
369
degraded to CO2 and NH3 (Jeong et al., 2017; Kim et al., 2017), with the uptake of O2.
370
Finally, when the exhaustion of decomposable organic fraction in the waste achieved, the
371
temperature starts decreasing in the composting pile and hence beginning of the
372
maturation phase observed and stabilization and humification of the organic matter occur,
14
373
producing a mature compost with humic characteristics (Ravindran and Sekaran, 2010).
374
Hence, this compost can be characterized as the stable and sanitized product that make the
375
composting for sustainable agriculture and resource management (Haug, 1993; Ravindran
376
et al., 2019; Gajalakshmi and Abbasi, 2008).
377
Over the last decades, researchers have been focusing on the study of the complex
378
interaction between physical, chemical and biological factors that takes place during
379
natural composting under controlled conditions. There are several factors affecting the
380
composting process, such as nutrient balance, pH, porosity and moisture of the
381
composting mixture, aeration, moisture content and temperature. Thus, the control of
382
these parameters such as, porosity, nutrient content, C/N ratio, temperature, pH, moisture
383
and oxygen supply have established to be key factors for optimization of composting
384
process as they determine the optimal conditions for organic matter degradation and
385
microbial growth (Agnew and Leonard, 2003; Das and Keener, 1996; Haug, 1993;
386
Richard et al., 2002). The effectiveness of compost material with regard to their beneficial
387
effects on agricultural as a nutrient source, depends on the quality of the compost. The
388
quality criteria for compost material are determined by their nutrient content, organic
389
matter stabilization, presence of heavy metals and toxic compounds.
390
The traditional composting process results in partial decomposition, which has
391
phytotoxic effects in the final product (Ravindran et al., 2014). In order to overcome the
392
conventional composting limitations, several researchers have employed an in-vessel
393
composting strategy for different types of waste such as fly ash with organic waste
394
(Mandpe et al., 2019); swine manure (Awasthi et al., 2019a; Yang et al., 2019; Ravindran
395
et al., 2019); municipal solid waste (Malinowski et al., 2019); sewage sludge (Jain et al.,
396
2019). An in-vessel composting system provides a controlled environment, which
397
eliminates some of the obstacles inherent in traditional composting systems for producing
398
nutrient rich matured manure. Despite that, environmental concerns related to composting
399
existed, and can be categorized into three aspects described below;
400
3.5.1. Residual pollutants from the contaminated feed stock’s
15
401
Due to the existing contaminants in the feedstock, the levels of residual pollutants
402
partly determined the feasibility of composting in field utilization. Such contaminants
403
include heavy metals, organic pollutants, and emerging contaminants. Heavy metals refer
404
to metallic chemical elements that had a higher density and 4.5 cm3. Due to the high
405
toxicity, long persistence, and bio-magnification traits, heavy metal accumulation in soils
406
have become a global issue (Yang et al., 2017). Feedstock for composting, such as animal
407
manure or sewage sludge, generally contain a certain amount of heavy metals, and these
408
metals would be concentrated in the compost with the degradation of an organic
409
compound, although the bio-availability is reduced (Li et al., 2012). For example, a
410
nationwide survey on heavy metal content in compost derived from animals manures
411
found the concentration of Zn, Cu, Cr, Ni, Pb, As, Co, Cd, and Hg ranged from 11.8-3692,
412
3.6-916, 0.7-6603, 0.7-73, 0.05-189, 0.4-72, 0.1-94, 0.01-8.7, and 0.01-1.9 mg kg-1,
413
respectively (Yang et al., 2017). There were even higher levels of heavy metals in other
414
feed stock’s like sewage sludge (Awasthi et al., 2017, 2018). Accordingly, government
415
across different countries have introduced strict standard to regulate the maximum
416
permissible values for organic compost.
417
Organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) such as
418
polychlorinated biphenyl (PCB), pesticides, and petroleum were generally found in
419
compost (Ren et al., 2018; Houot et al., 2012). Even though the concentrations of such
420
pollutants were generally reduced through composting via biodegradation, the
421
concentration of residual organic pollutants might be higher than the background levels,
422
which means inputs of such compost should be limited (Brandli et al., 2007). The other
423
residues pollutants are antibiotic resistance genes and emerging contaminants such as
424
micro particle, hormone, and perfluorinated compounds (PFCs). There were also high
425
content of antibiotic in the common feedstock’s of composting (i.e., sludge and manures),
426
and studies focused on the fate of antibiotics and antibiotics resistance genes have become
427
a research hotspot since 2009 (Ezzariai et al., 2018). However, studies on the
428
transformation of antibiotic resistance genes are still limited, and the obtained results are
16
429
partly contracted with each other (Wu et al., 2011; Wang et al., 2015). Therefore, more
430
detailed research on the fate of antibiotic residence genes during composting should be
431
further investigated.
432
3.5.2. Odors
433
During composting, series of stinky gases such as hydrogen sulfide, volatile organic
434
sulfides, ammonia, pyridine, amines, hydrocarbons, terpenes, alcohols, ketones,
435
aldehydes, and esters were generated (Wei et al., 2017). Additionally, the constitution and
436
concentration of different compounds varied significantly and affected by the feed stock
437
properties, composting operation conditions, and the ambient environment (Awasthi et al.,
438
2018). Even though their concentrations are quite low, emission of odors gases can lead
439
to significant allergy among the neighbor. Fortunately, we can significantly reduce the
440
emission of odors nowadays by mediating ingredient properties, enhancing aeration,
441
providing catalysts and inoculant, and building airtight composting facilities. More
442
specifically, providing enzymatic catalysts on the surface of a compost pile or in the
443
airspace above it can facilitate the biological reaction and eventually degrade the odorous
444
compounds. Additionally, the addition of straw materials, chemical reagents (such as
445
FeCl3, lime, and MgO), and biochars can effectively prevent the emission of odors during
446
composting (Ren et al., 2018; González et al., 2019).
447
4. Challenges and prospects of bio-based circular economy
448
The term “bio-based circular economy” can refer to an approach that promotes the
449
reuse and valorization of organic wastes and residues. In the recent years, it has more
450
essential options for the conversion of bio-waste and their intermediate products into
451
bioenergy or secondary raw materials (Mohan et al., 2016).
452
4.1. Integrated organic solid waste treatments technologies
453
Rapid urbanization is surely creating enormous pressure on the urban bodies to
454
deal with the demands of the developing populace and the consistently expanding huge
455
amounts of waste all through the world (Awasthi et al., 2017; Zhou et al., 2014; Zhao et
17
456
al., 2014). Because of lack of information and inappropriate waste gathering frameworks
457
the untreated waste eventually arrives at the landfills and are responsible of aggravating
458
the natural conditions. Inability in completely grasping all the issues identified with the
459
management of municipal solid waste (MSW) ends up being one of the most challenging
460
issues of degrading the environment. MSW contains a significant portion (30–50%) of
461
organics (Bhattacharyya et al., 2008). The amount of each waste stream within the total
462
solid waste generated varies with changes in specific parameters (Arafat et al., 2013).
463
At present, waste is viewed as an important asset rather than recognizing it to be
464
useless and resource extraction from them is one of the primary goals of sustainable waste
465
management (Kumar, 2017). A few treatment technologies are feasible with varying waste
466
management abilities (Arafat et al., 2013). However, no single technique can fix the issue
467
of waste management (Tehrani et al., 2009). Therefore, an integrated approach towards
468
the treatment of solid waste is of great importance. It not just gives an adaptable waste
469
treatment decision (organic or inorganic) but energy and asset recovery can likewise be
470
effectively accomplished through it (Zhou et al., 2014). The significant reason behind
471
treating the organic waste is to recover recyclables and energy from it and furthermore to
472
improve the quality of the waste handling it (Figures 4 and 5).
473
Organic solid waste treatment technologies may incorporate either a combination
474
of mechanical, thermal or biological treatments or any of the three separately. The
475
significant ones being the thermal and biological treatment techniques (Kumar, 2017). An
476
integrated approach is certainly required which should be from its generation to its final
477
disposal. The treatment procedure chosen ought to be such that it accomplishes the
478
objectives of waste management. When the waste is explicitly organic in nature the
479
biological procedures plays the main job, which are the aerobic and anaerobic digestion,
480
bio-landfilling, bio-drying and vermicomposting. Aerobic digestion transforms the easily
481
degradable organic waste aerobically into soil conditioners and carbon dioxide with
482
resulting heat production (Polprasert, 2007). Whereas anaerobic digestion involves
483
degrading the organic waste anaerobically which leads to recovery of energy in form of
18
484
biogas (Tchobanoglous et al., 2004). In this process, the organic waste gets decomposed
485
by different microbial actions. However, in comparison to aerobic digestion, anaerobic
486
digestion produces less solid sludge (Henze et al., 2008). Bio-landfilling is another
487
procedure utilized for managing the solid waste that upgrades the waste degradation and
488
also increases the measure of biogas formation (Davis and Cornwell, 2008). Anaerobic
489
processing and bio-landfilling varies just in the utilization of reactor type which is simply
490
the landfill itself during bio-landfilling. Bio drying is another such approach that aides in
491
diminishing the measure of water present in the natural waste which would some way or
492
another hamper the energy recuperation process (Polprasert, 2007). It additionally helps
493
in saving energy for usage in residue derived fuels (Adani et al., 2002). All the major
494
integrated organic waste treatment technologies discussed above have been shown in
495
Figure 5. All these technologies require a legitimate identification and assessment of
496
accessible options, planning, collection, isolation, treatment and final disposal. In addition,
497
understanding the potential interrelationships between various treatment options would
498
help in building up an integrated approach where all the individual techniques supplement
499
one another.
500
4.2. Techno-economic feasibility of aerobic and anaerobic digestion technologies
501
“Wastes from one industrial process can serve as the raw materials for another,
502
thereby reducing the impact of industry in the environment” was described by Frosch and
503
Gallopoulos (1989). Eco-industrial parks are making collaborate with other inter-company
504
to optimize the resource efficiency. They can share the resources, water, and energy.
505
Whereas working together, the community of businesses looking for collective benefits
506
rather than individual benefits. Hence, the eco-industrial parks make industrial symbiosis
507
for improvement of their economic status by sharing the raw materials and wastes. An
508
anaerobic digestion process is a process, that can biochemically modify the whole biomass
509
into specific components and this process can produce carbon dioxide, phosphorus and
510
ammonia to improve the economic and environmental benefits. The biochemical
511
processes in anaerobic digestion are nature. It is accepted and established as an innovative
19
512
technology to produce the bio energy from whole biomass (Kavitha et al., 2017; Stiles et
513
al., 2018).
514
An assessment of techno-economic analysis and life cycle are essential for any
515
processes to be sustainable, which can be analysis by three parameters like economical
516
viability, technical feasibility and environmental sustainability. There are number of
517
industries and investors looking the production of biogas using anaerobic digestion
518
method for generation of energy and a high-risk investment, because of its process like
519
feedstock supply, segregation of solid waste, characteristics features of feedstock, profit
520
issues and optional problems (Schmidt, 2014). Based on the above facts, the many
521
governments have support and encourage by provide the funds to company for this area.
522
According to Waste Management World (2017), the global biogas market will reach $50
523
billion on 2026, hence it is crucial for scientist and researchers to conduct their research
524
on anaerobic digestion at systems level, as well as techno-economic and environmental
525
benefits perspective. So, it is very essential to assess integrated analyses of economic
526
viability and technical feasibility on anaerobic digestion technologies. The economic
527
analysis for particular process is analyzed by values of materials for construction, interest
528
rate and labor cost (Murthy, 2016). The techno-economic analysis and life cycle
529
assessment (LCA) are providing the insights on the feasibility of a process and it is very
530
important for bioenergy and bio-based products (Murthy, 2016). Techno-economic
531
evaluation determines that maximum funds spent in biogas purification for various
532
applications including the power-to-gas, electricity and vehicle fuel. Various anaerobic
533
digester upgrading methods are available with membrane separation, water scrubbing,
534
pressure adsorption and solvent scrubbing. Hence, the government has high cost for
535
upgrading the anaerobic systems (Bauer et al., 2013; International Energy Agency, 2015).
536
Anaerobic co-digestion process is gradually more used to change the liquid and
537
solid waste into nutrient rich content and reduce the impact of noxious compounds in the
538
process and enhance the yield of biogas (Carlini et al., 2017). It is considered as the most
539
important method for the management of biomass to production of biogas, and the
20
540
digestate can be used as bio-fertilizer. This process can completely eliminate the
541
pathogens (Sarsaiya et al., 2019b; Sarsaiya et al., 2019c) and to prevent the emission of
542
greenhouse gases (Torquati et al., 2014; Weiland, 2010). United Kingdome and Hong
543
Kong have strong-willed to sponsor the use anaerobic digestion methods for energy
544
production and recovery. It also fulfils the Sustainable Development Goals (SDGs) for
545
development of renewable energy (HK Policy address, 2017). In Italy, there are 35% of
546
biogas plants that produce 70 – 100 MW electricity, which straight relate to government
547
incentives (Gestore Servizi Energetici, 2015).
548
During the techno-economic analysis, three areas of anaerobic digestion process
549
are important: (1) unit operations with collection and transport of feed stocks, (2) to
550
provide the treatment facility for production of biogas, and (3) upgrading the bio-energy
551
for various applications as electricity and liquid methane for household cooking. There is
552
20-50% of budget allotted to collect and transport the waste by manual collection or
553
trucks/trailers in the sector of municipalities and corporations (Daniel and Perinaz, 2012).
554
In Ireland, cost of $22 – 35/month was spent to waste collection from various sectors like
555
houses and industries for anaerobic digestion (Koushki et al., 2004). Figure 2 shows the
556
cost associated with complete value chain of anaerobic digestion for solid waste.
557
4.3. Proposed models for a circular economy
558
4.3.1. Co-digestion
559
Biodegradation of more than one type of waste in one anaerobic bioreactor, albeit
560
in appropriate proportions, is a promising technique for increasing product yields ascribed
561
to the balance of nutrients that enhances microbial performance coupled with the
562
economic benefits of treating a variety of materials in a single facility (Abad et al., 2019;
563
Awasthi et al., 2019b; PagésDíaz et al., 2011). Several researchers have observed
564
improvement of AD systems from a number of solid wastes demonstrating that anaerobic
565
co-digestion, illustrated in Figure 3a, could be a favorable method for achieving circular
566
economy.
21
567
A study by Panichnumsin et al. (2010) reported that an increase of 41% in the
568
specific methane yield would be realized when a cassava pulp/pig manure mixture was
569
applied in the ratio of 3:2 relative to the use of pig manure alone. The potential cause of
570
the improved performance was the improved process stability as well as an increase in the
571
amounts of easily degradable macromolecules (Panichnumsin et al., 2010). In another
572
study, improved buffering capacity and suitable carbon-to-nitrogen ratio were found to
573
facilitate a stable AD process at high organic load even without regulation of pH when
574
food waste and cattle manure were in ratio of 2:1 leading to an increase in the methane
575
production by 55.2% compared to food waste alone (Zhang et al., 2013). The co-digestion
576
strategy is also beneficial when targeting other AD bio-products such as H2 and VFAs. A
577
research by Zhou et al. (2013b) examined several mixtures containing food waste, primary
578
sludge and waste activated sludge and concluded that co-digestion of the substrates
579
improved hydrogen production compared to single substrates. The optimal increase was
580
realized when a food waste/ primary sludge/waste activated sludge mixture was applied
581
at a ratio 80:15:5 (Zhou et al., 2013b). The impact of co-digesting waste activated sludge
582
and corn straw was investigated by Zhou et al. (2013a). They observed an improvement
583
in the maximum VFAs yield reaching up to 69% when a ratio of 1:1 of the mentioned
584
substances was used compared to sludge only in addition to the manipulation of the
585
product profile. Further results indicated that protein and carbohydrate consumption were
586
enhanced by more than 120 and 220%, respectively (Zhou et al., 2013a).
587
Besides the conventional wet digestion, solid wastes can also be biodegraded at
588
high total solids content of between 15 – 40% in the so-called dry AD processes using e.g.
589
horizontal plug flow, sequential batch, textile-based, and vertical plug flow reactors. Dry
590
AD is a promising technology for regions with low water availability and might also evade
591
the pretreatment step for recalcitrant materials in addition to requiring more compact
592
bioreactors (Awasthi et al., 2019a). An increase in methane yield of 14% was achieved
593
when citrus wastes, chicken feather and wheat straw at a ratio of 1:1:6 in dry batch AD
594
essays compared to single substrates. When the same substrate ratio was used in a
22
595
continous plug flow bioreactor, a yield of 362 NmLCH4/gVSadded at a loading rate of 2
596
gVS/L/d was obtained (Patinvoh et al., 2018). Another dry AD process was demonstrated
597
by Capson-Tojo et al. (2017) in which food waste and cardboard were co-digested in batch
598
mode. Their research focused on the impact inoculum-to-substrate ratio which was found
599
to have a significant influence on the methane yield, microbial dynamics as well as the
600
productions of hydrogen and VFAs.
601
4.3.2. Recovery of multiple products
602
The AD process can be employed for the recovery of more than a single metabolite
603
at reasonable yields, thereby increasing the process conversion efficiency (Figure 3b and
604
5). One of the possible scenarios was recently successfully executed involving the
605
production of both VFAs and biogas from urban biowastes (organic fraction of municipal
606
solid waste and waste activated sludge) in a pilot two-stage AD process (Valentino et al.,
607
2019). The acidification reactor in the first stage operated at 37 °C generated VFAs up to
608
at 19.5 g COD/L without external pH control. In the second stage, the solid-rich fraction
609
collected from the acidification reactor was converted into biogas containing 63–64% CH4
610
at a yield of up to 0.40 m3/kg VS at 37 °C. Furthermore, it was claimed that a similar
611
scaled-up system at the mentioned temperature would be thermally sustainable leading to
612
a surplus of thermal energy in addition to an estimated revenue stream of 609,605 €/year
613
from electricity (Valentino et al., 2019). Another scenario would involve the co-
614
production of lactic acid and methane. Using organic fraction of municipal solid waste,
615
Dreschke et al. (2015) designed a process to produce lactic acid in batch reactors at 37 °C.
616
Lactic acid made up to 83% of the acid content with a yield of up to 37 g/kg was reached
617
after 24 hours. They also reported 75% Lactobacilli enrichment in the microbial
618
consortium. The effluent from the batch fermenters were then co-digested with digested
619
municipal sludge and a methane yield of up to 618 NmL CH4/gVS was attained. In
620
addition, the final effluent was proposed as a potential source of organic fertilizer
621
(Dreschke et al., 2015).
23
622
In addition to the production of dissolved metabolites, it has been demonstrated
623
the AD process can be applied for biosynthesis of both hydrogen and methane. The system
624
can be optimized to yield the separate products in two distinct stages to produce a mixture
625
known as hythane, which usually contains 10–25% hydrogen in methane (Liu et al., 2013;
626
Wang et al., 2011). In addition to the possibility of effectively optimizing the separate
627
processes, the other advantages of hythane production, relative to the single hydrogen or
628
methane production, include the reduced overall fermentation time and the enhanced
629
energy recovery (Liu et al., 2013). For instance, Siddiqui et al. (2011) observed that a
630
single phase AD reactor resulted in 0.48 m3/kgVSdestroyed methane yield, while a two-phase
631
process produced 0.67 m3/kgVS destroyed methane in addition to hydrogen at a yield of
632
0.13 m3/kgVS destroyed. Elsewhere when pulp and paper sludge and food waste (on a 1:1
633
mixing ratio based on the VS) were used, Lin et al. (2013) demonstrated a two-stage
634
process in which hydrogen was produced in mesophilic conditions in stage one while the
635
digestate from this stage was subjected to thermophilic AD conversion. They reported no
636
accumulation of VFAs in the first reactor that yielded hydrogen up to 64.48 mL/gVSfed at
637
pH levels 4.8–6.4 while the methane yield was 432.3 mL/gVSfed at pH levels 6.5–8.8 in
638
the second reactor. In another research, Wang et al. (2011) co-digested cassava stillage
639
and excess sludge in thermophilic conditions and found a 25% higher energy yield when
640
a two-phase fermentation system was carried instead of a one phase system. By mixing
641
cassava stillage and excess sludge at ratio of 3:1 (on VS basis) a hydrogen yield of 74
642
mL/gVSfed and a methane yield of 350 mL/gVSfed were attained.
643
4.3.3. Combined bioprocesses
644
Another strategy to implement systems that potentially meets the circular economy
645
criteria involves combination of processes that are independently optimized to recover
646
renewable energy, chemicals and materials (Figure 3c). Successful demonstrations of such
647
schemes mainly comprise established industrial processes involving the production of
648
ethanol from lignocellulosic materials as the major activity. For instance, wheat straw was
649
converted into three important products: ethanol, hydrogen and biogas (Kaparaju et al.,
24
650
2009). The integrated bioprocess began with hydrothermal pretreatment of the wheat
651
straw which resulted into cellulose-rich solid fraction and a hemicellulose-rich liquid
652
fraction. The solid fraction was applied for ethanol fermentation in which a yield of up to
653
0.41 g/g-glucose was realized. Biogas was then produced using the effluent from the
654
ethanol production processes yielding up to 0.324 m3 CH4/kg VSadded. The liquid fraction
655
from the straw pretreatment was separately used in dark fermentation reactors and yielded
656
up to 178.0 mL H2/g-sugars. Similarly, the effluent from the hydrogen production
657
processes was anaerobically digested to produce biogas at a yield of up to 0.381 m3
658
CH4/kg VSadded. Woody biomass has also been applied as a feedstock for production of
659
both ethanol and biogas. Safari et al. (2016) during an investigation on the performance
660
of dilute H2SO4 pretreatment of softwood pine demonstrated the potential improvements
661
in the ethanol and biogas productions. Similar to the previously described work, the solid
662
fraction from the pretreatment, consisting mostly of cellulose, was applied to produce
663
ethanol production while the liquid fraction consisting of sugars was applied to produce
664
biogas. The authors reported the highest ethanol yield of 53% while the highest biogas
665
production yield was 162 m3 methane/ton of pinewood. Another key outcome from their
666
experiments showed that the inhibitory compounds released at the pretreatments at 180
667
°C had more negative impact on the production of biogas compared to the production of
668
ethanol (Safari et al., 2016).
669
Another study was made by Wachendorf et al. (2009) proposed the use of silage
670
for production of biogas and processing of a solid fuel as a potential feed stock for
671
combustion or gasification. The silage was first subjected to hydrothermal conditioning
672
and then mechanically dehydrated to separate the liquid from the solid portions. The
673
former portion was used as substrate for production of biogas while the latter portion
674
remained as a press cake (solid fuel). Moreover, the digestate from the anaerobic digester
675
could be used as fertilizer for growing more silage, making the proposed scheme highly
676
efficient in resource recovery (Wachendorf et al., 2009). In addition, a promising model
677
that could be appropriate for agricultural wastes would be the simultaneous recovery of
25
678
essential materials such as edible fungi. Nair et al. (2018) demonstrated an integrated
679
bioprocess in which biogas, ethanol and protein-rich fungal biomass from pretreated
680
wheat straw. Neurospora intermedia was the species applied for producing ethanol
681
(yielding up to 90% of the theoretical maximum) as well as the biomass. Further results
682
revealed that compared to untreated straw, an increase of up to 162% in the methane yield
683
would be realized from the anaerobic treatment of the liquid stream from the pretreatment
684
process. Besides, co-digestion essays of the mentioned stream with thin stillage, a waste
685
stream from the ethanol production process, resulted in an increase in the total energy
686
output of up to 27% (Nair et al., 2018).
687
5. Conclusion
688
This review paper evaluated resource recovery and circular economy options from
689
organic solid waste using aerobic and anaerobic digestion technologies. Utilization of
690
organic waste residues would elevate biorefinery concept and social acknowledgments.
691
Aerobic and anaerobic digestion are widely acceptable technologies to enhance the
692
efficiency for energy and other high value bio-based products production. Overall, various
693
advanced models have recently been proposed and already worked out in line with a
694
holistic biorefinery avenue that bring enormous industrial importance. Hence, these
695
technologies should be critically assessed as an interracial pyramid scheme and a techno-
696
economic feasibility examination of this plan is also needed.
697
Acknowledgments
698
This work was supported by a research fund from the International Young
699
Scientists from National Natural Science Foundation of China (Grant No. 31750110469),
700
China; the Shaanxi Introduced Talent Research Funding (A279021901), and The
701
Introduction of Talent Research Start-up Fund (Grant No. Z101021904), College of
702
Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi
703
Province 712100, China. The Mobility for Regional Excellence-2020 (Grant No. RUN
704
2017-00771), European Union's Horizon 2020 Research and Innovation Programme
26
705
under the Marie Skłodowska-Curie grant agreement No. 754412 at University of Borås,
706
Borås, Sweden, and Guizhou Science and Technology Corporation Platform Talents Fund,
707
China (Grant No.: [2017]5733-001 & CK-1130-002). We are also thankful to all our
708
laboratory colleagues and research staff members for their constructive advice and help.
709
References
710
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1148
Figure captions:
1149
Figure 1. Potential feed stock, conversion technologies and products
1150
Figure 2. Biogas production and the potential applications.
1151
Figure 3. Proposed models for circular economy using anaerobic digestion technology:
1152
co-digestion using municipal, industrial and agricultural wastes (a), recovery of multiple
1153
products (b) and integration of bioprocesses (c).
1154
Figure 4. Schematic presentation of integrated food waste and wastewater treatment
1155
method (MOWFAST) and conventional sludge (PSSS) treatment by anaerobic digestion
1156
(Extracted from Kaur et al., 2019)
42
1157
Figure 5. Various costs associated with the value chain of waste treatment from collection
1158
to products generation. (Extracted from Rajendran and Murthy, 2019).
1159
Figure 6. Different technologies and processes for the utilization of organic solid waste
1160
for nutrients recovery.
1161 1162 1163 1164 1165
1166 1167
Figure 1: Potential feed stock, conversion technologies and products
43
1168
1169 1170
Figure 2: Biogas production and the potential applications
44
1172 1173 1174 1175
Figure 3: Proposed models for circular economy using anaerobic digestion technology: co-digestion using municipal, industrial and agricultural wastes (a), recovery of multiple products (b) and integration of bioprocesses (c)
45
1176 1177
Figure 4: A Schematic presentation of integrated food waste and wastewater treatment
1178
method (MOWFAST) and conventional sludge (PSSS) treatment by anaerobic digestion
1179
(Extracted from Kaur et al., 2019)
46
1180 1181
Figure 5: Various costs associated with the value chain of waste treatment from collection
1182
to products generation (Extracted from Rajendran and Murthy, 2019)
47
1183 1184
Figure 6: Different technologies and processes for the utilization of organic solid waste
1185
for nutrients recovery
1186
148.
1187
Tables captions:
1188
Table 1. Wastes production by economic and household activities until 2016 on
1189
percentage basis.
48
1190
Table 2. Potential organic waste substrates and optimal conditions for hydrogen
1191
production.
1192
Table 3. Potential organic waste substrates and optimal conditions for production of
1193
volatile fatty acids.
1194 1195 1196 1197
49
1198
Table 1. Countries
Mining and quarrying
Manufacturing
Energy
Constru ction and demoliti on
Other economic activities
Households
EU-28 Belgium Bulgaria Czechia Denmark Germany Estonia Ireland Greece Spain France Croatia Italy Cyprus Latvia Lithuania Luxemburg Hungary Malta Netherland s Austria Poland Portugal Romania Slovenia Slovakia Finland Sweden United Kingdom Iceland Liechtenste in Norway
25 0 82 1 0 2 26 16 78 16 1 12 0 5 0 1 0 1 8 0
10 23 3 18 5 14 37 35 6 11 7 8 17 33 19 41 7 17 1 10
3 1 8 4 4 3 25 2 4 3 0 2 2 0 11 2 0 16 0 1
36 31 2 40 58 55 5 10 1 28 69 24 33 36 4 8 75 23 69 70
16 36 3 23 16 17 6 28 4 26 14 31 29 10 30 32 11 25 13 13
8 8 2 14 17 9 2 10 7 17 9 22 18 16 34 17 6 18 8 6
0 39 3 87 0 3 76 77 6
9 17 17 4 26 32 8 4 4
1 11 1 4 14 9 1 1 0
73 10 12 0 10 9 11 7 49
10 18 35 3 38 29 3 7 30
7 5 33 2 12 18 1 3 10
0 3
25 2
0 0
4 88
31 1
40 5
3
14
2
27
32
22
50
1199
Montenegr o North Macedonia Serbia Turkey Bosnia and Herzegovin a Kosovo
19
2
18
37
10
13
49
51
0
0
0
0
79 11 2
3
1
2
27
12 26 71
0
0
3 37 0
14
20
40
6
10
11
Source: Eurostat (online data code: env_wasgen)
1200 1201 1202
Table 2. Substrate
Operating conditions
Hydrogen production
Reference
performance Melon waste
Continuous fermentation
Yield= 352 mL/gVSadded
T= 55 °C
(Akinbomi and Taherzadeh, 2015)
HRT= 5 days pH= 5.0 Melon waste
Batch essay
Productivity= 351.12
T= 36 °C
mLH2/Lreactor.h
(Turhal et al., 2019)
pH= 5.5 – 6.0 Apple waste
Continuous fermentation
Yield= 635 mL/gVSadded
T= 55 °C
(Akinbomi and Taherzadeh, 2015)
HRT= 5 days pH= 5.0 Mixed food waste
Continuous fermentation
Yield= 1.80mol-H2/mol-
T= 55 °C
hexose
(Shin et al., 2004)
HRT= 5 days pH= 5.5 Mixed food waste
Batch essay
Yield= 176.10 mL
(Wongthanate and
T= 55 °C
H2/gCOD
Mongkarothai, 2018)
Initial pH= 7.0
51
Waste peach pulp
Batch essay
Productivity = 35.6
T= 37 °C
mLH2/h
(Argun and Dao, 2017)
Initial pH= 5.9 Fruit and vegetable waste
Batch essay
Yield= 76 mL H2/g VS
(Keskin et al., 2018)
Continuous fermentation
Production rate = 7.4
(Han et al., 2016)
T= 55 °C
LH2/L/d
T= 55 °C Initial pH= 7.0 Waste bread
HRT= 6 hours pH˃ 4.0
1203
52
1204
Table 3. Solid waste
Optimal conditions
Production performance
Reference
Olive mill solid waste
Batch-fed fermentation
Concentration=3.69
(Cabrera et al., 2019)
T= 35 °C
gCOD/L
pH= 9.0 Mixed food waste
Semi-continuous
Yield= 0.54 gVFA/ VSfed
(Wainaina et al., 2019a)
Yield = 0.39 gVFA/ VSfed
(Lim et al., 2008)
Yield = 0.46 gCOD/gCOD
(Garcia-Aguirre et al.,
fermentation * T= 37 °C HRT= 6.67 days OLR= 2 gVS/L/d pH= 3.6 – 4.1 Mixed food waste
Semi-continuous fermentation T= 35 °C HRT= 12 days pH= 5.5
Meat and bone meal
Batch fermentation T= 55 °C
2017)
pH= 10.0 Organic fraction of
Continuous fermentation
Yield = 0.31
municipal solid waste
T= 55 °C
gCOD/gCODin
HRT= 3.3 days OLR= 20.5
(Valentino et al., 2018)
VFA concentration=16.0
kgVS/m3/
d
gCOD/L
pH ≥ 4.0 Waste activated sludge
Batch fermentation
Concentration=2708.02
T= 21 °C
mg/L
pH= 10.0
1205
* A membrane bioreactor was used for simultaneous fermentation and product recovery
1206 1207 1208 1209
53
(Chen et al., 2007)
1210 1211 1212 1213
149.
1214
Highlights:
1215 1216
Reviewed resource recovery from organic solid waste towards bio-circular economy.
1217 1218
Aerobic and anaerobic digestion technology: a sustainable approach for biorefinery.
1219
Organic solid waste: a potential biorefinery and bio-economy option.
1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239
150. Steven Wainainab, Dr. Steven is write 60% part of this manuscript Mukesh Kumar Awasthia*,b, He has design and prepared the content of review and also write 30% part of this manuscript Surendra Sarsaiyaf, He has help to prepared the figures Hongyu Chend, He has help to write introduction and prepared table 1. Ekta Singhc, He has put some contribution to arranged and crossed check references Aman Kumarc, He has put some contribution to arranged and crossed check references B. Ravindrane, He has put some contribution to write nutrient recover section Sanjeev Kumar Awasthia, He has put some contribution to arranged and crossed check references Tao Liua,
54
1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252
He has put some contribution to arranged and crossed check references Yumin Duana, He has put some contribution to arranged and crossed check references Sunil Kumarc, Dr. Kumar is very help to improve the manuscript quality and English Zengqiang Zhanga, Prof. Zhang is very help to improve the manuscript quality Mohammad J. Taherzadehb Prof. Mohammad is very help to improve the manuscript quality and English
151. Declaration of interests
1253 1254 1255 1256
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
1257 1258 1259 1260
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
With warm regards,
Dr. Mukesh Kumar Awasthi
Associate Professor
1261 1262 1263
College of Natural Resources and Environment, Northwest A&F University, Taicheng Road 3#, Yangling, Shaanxi 712100, China.
55
1264 1265
152.
56
S0960-8524(20)30047-X https://doi.org/10.1016/j.biortech.2020.122778 BITE 122778
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
18 November 2019 6 January 2020 7 January 2020
Please cite this article as: Wainaina, S., Kumar Awasthi, M., Sarsaiya, S., Chen, H., Singh, E., Kumar, A., Ravindran, B., Kumar Awasthi, S., Liu, T., Duan, Y., Kumar, S., Zhang, Z., Taherzadeh, M.J., Resource Recovery and Circular Economy from Organic Solid Waste using Aerobic and Anaerobic Digestion Technologies, Bioresource Technology (2020), doi: https://doi.org/10.1016/j.biortech.2020.122778
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1
Resource Recovery and Circular Economy from Organic Solid Waste using
2
Aerobic and Anaerobic Digestion Technologies
3
Steven Wainainab, Mukesh Kumar Awasthia*,b, Surendra Sarsaiyaf, Hongyu Chend, Ekta
4
Singhc, Aman Kumarc, B. Ravindrane, Sanjeev Kumar Awasthia, Tao Liua, Yumin Duana,
5
Sunil Kumarc, Zengqiang Zhanga, Mohammad J. Taherzadehb
6
a
7
Shaanxi Province 712100, PR China
8
bSwedish
9
cCSIR-National
College of Natural Resources and Environment, Northwest A&F University, Yangling,
Centre for Resource Recovery, University of Borås, 50190 Borås, Sweden Environmental Engineering Research Institute, Nehru Marg, Nagpur -
10
440 020, Maharashtra, India
11
dInstitute
12
eDepartment
13
Gu, Suwon, Gyeonggi-Do, 16227, South Korea
14
fKey
15
Ethnomedicine of Ministry of Education, Zunyi Medical University, Zunyi, Guizhou,
16
China
17
*Corresponding author:
18
Dr. Mukesh Kumar Awasthi
19
E-mail: [email protected] (M.K. Awasthi)
of Biology, Freie Universität Berlin Altensteinstr. 6, 14195 Berlin, Germany of Environmental Energy and Engineering, Kyonggi University Youngtong-
Laboratory of Basic Pharmacology and Joint International Research Laboratory of
20 21
1
22
Resource Recovery and Circular Economy from Organic Solid Waste using
23
Aerobic and Anaerobic Digestion Technologies
24
Steven Wainainab, Mukesh Kumar Awasthia*, b, Surendra Sarsaiyaf, Hongyu Chend, Ekta
25
Singhc, Aman Kumarc, B. Ravindrane, Sanjeev Kumar Awasthia, Tao Liua, Yumin Duana,
26
Sunil Kumarc, Zengqiang Zhanga, Mohammad J. Taherzadehb
27
Abstract
28
With the inevitable rise in human population, resource recovery from waste stream is
29
becoming important for a sustainable economy, conservation of the ecosystem as well as
30
for reducing the dependence on the finite natural resources. In this regard, a bio-based
31
circular economy considers organic wastes and residues as potential resources that can be
32
utilized to supply chemicals, nutrients, and fuels needed by mankind. This review
33
explored the role of aerobic and anaerobic digestion technologies for the advancement of
34
a bio-based circular society. The developed routes within the anaerobic digestion domain,
35
such as the production of biogas and other high-value chemicals (volatile fatty acids) were
36
discussed. The potential to recover important nutrients, such as nitrogen through
37
composting, was also addressed. An emphasis was made on the innovative models for
38
improved economics and process performance, which include co-digestion of various
39
organic solid wastes, recovery of multiple bio-products, and integrated bioprocesses.
40
Keywords: Resource recovery; organic solid waste; circular economy; anaerobic
41
digestion; composting.
42
2
43
1. Introduction
44
Disposal of organic solid waste (OSW) has become one of the hot topics in research.
45
Simultaneously, the collection and recycling of OSW is presently principal environmental
46
concerns in both urban and countryside areas in numerous industrial and developing
47
countries (Vinay et al., 2018). With the rapid development of modern society and the
48
continuously rising population all over the world, the generation of OSW is projected to
49
be significantly increased annually. Statistically, 2.01 billion tons of municipal wastes
50
were produced worldwide and by 2050, approximately 3.4 billion tons of municipal
51
wastes will be emerged annually if the current trend continues (Luis et al., 2019).
52
Furthermore, improper waste management is not only pernicious to human beings and
53
local environment, but also exacerbates the climate change and causes the sanitation
54
system thus forcing nations and governments to invest more financial and material
55
resources for its remediation. Consequently, ideal technologies to OSW management is
56
not only conducive to environmental protection and sustainable development but also
57
critical to forge a circular economy (Felix et al., 2019).
58
In a circular economy, recycling and reuse is the major principle for designing and
59
optimization of products. As nations and authorities all over the world gradually
60
embracing the concept of circular economy and attempting to bring it in practice,
61
disposing wastes with an advanced and sustainable way could promote efficient economic
62
growth with reduced environmental impacts at the same time (Pooja et al., 2018;
63
González-García et al., 2019). This concept not only creates more job opportunities but
64
also able to converts waste into treasure and transforms it to a new clean energy and
65
material sources through classified management and recycling. Governments and
66
municipalities around the world have started paying great attention to it and constantly
67
reform the urban wastes management system, making it an important part of the green
68
economy and sustainable development (Biswabandhu et al., 2019).The majorities of OSW
69
are composed of animal wastes, municipal and agricultural wastes, thus OSWs are
70
generally rich in nutrient substances, such as proteins, minerals, and sugars, which can be
3
71
reused as raw materials in the production of green energy (Kiros et al., 2017). Nevertheless,
72
due to the restrictions of the allocated budgets, infrastructure development, OSW
73
management is still suffering from improper treatment in most developing and even in
74
few developed countries. Therefore, the most decisive factors in OSW management is to
75
develop an economically sustainable process followed by technically feasible, socially
76
and legally acceptable as well as environment friendly. Hence, OSW management is
77
usually one of the most important challenges for the authorities and a hot research topic
78
all over the world (Alejandra et al., 2018). Based on the characteristics of the OSW, they
79
are generally treated to be converted into organic fertilizer or directly transport to landfill
80
sites or incineration facilities. Therefore, the conventional methods including landfill,
81
incineration, and composting are traditional matured technologies for OSW management.
82
Different processing technologies possess varied advantages and also drawbacks as well.
83
On the other hand, biological technologies for OSW treatment are now-a-days seriously
84
considered in the world because of the characteristics of low energy consumption, low
85
cost and investment, high organic removal rate and meeting the requirement of circular
86
economy (Nurul et al., 2018). However, due to the different economical possibilities of
87
various countries and the OSW treatment technologies, traditionally, the most commonly
88
applied technologies are still landfill in developing countries and incineration in most of
89
the developed countries.
90
Composting and anaerobic digestion (AD) depend on the decomposition and
91
degradation of organic matter in OSW to stable substances, such as humus and digestate
92
could be applied to soil as a fertilizer or soil improvement if their qualities are at standard
93
level. During the AD process, biogas that is rich in methane and carbon dioxide are
94
produced, which could be used as a fuel for combustion in transport or energy production
95
(Nuhaa, 2019). Meanwhile, though these two technologies are not new, they probably
96
could have a key role in achieving circular economy objectives to divert OSW from
97
landfill and incineration and improving the circularity of biological nutrients. However,
98
there are still many practical and technical issues that need to be addressed in the
4
99
application of these two biological treatment methods (Zhang et al., 2019). In Europe, 60
100
million tons of OSW could be potentially recycled by AD and composting technologies
101
(Felix et al., 2019). This would save about one million tons of nitrogen and 20 million
102
tons of organic carbon (Luis et al., 2019). At present, most of these nutrients are lost
103
through landfilling organic waste. The European Commission reported that the European
104
countries in average recycle only 5% of the total OSW. It is estimated that if a higher
105
portion of OSWs could be recycled and reused, it could replace approximately 30% of the
106
chemical fertilizer applied to soil, that means 1.8 million tons phosphate fertilizer every
107
year (Luis et al., 2019).
108
OSW can be converted to vary products through different processing technologies,
109
such as fertilizer, heat, clean fuel from AD, plant and soil food from composting as well
110
as methane from landfill. Despite this, either composting or AD must be fully understood
111
prior to their wider deployment. This review comprehensively describes the current
112
generation state of the OSW and different disposal technologies, particularly for
113
composting and AD and elaborating on the details of their treatment process and their
114
environmental, social and economic applicability. Both of these two technologies are not
115
completely economically feasible, because of which they have not been successful applied
116
in many countries. A deep insight has been attempted in this paper to make a better
117
understanding for the importance of its economic viability.
118
2. Organic solid waste generation
119
The constant increase of worldwide population to more than seven billion plus the
120
continuous improvement of global welfare has resulted in a rapid increase of the daily
121
consumption of resources and products, thus tremendous amount of recyclable wastes is
122
generated (Alejandra et al., 2018). The generation of wastes per capita in most of the
123
industrial countries are higher than developing countries, which is most likely connected
124
to the socio-economic status of the country. Taking the EU as an example, EU-28 nations
125
produced nearly 25.38 million tons of wastes through all the activities including economic
126
and households in 2016. Amongst it, the specific waste output of each EU nation also
5
127
illustrated that the total amount of wastes generated had a certain degree of negligible
128
relationship with the economic scale and population of a nation (Forough et al., 2019). It
129
can be clearly observed from Table 1 that the EU member states with the smallest
130
population and area possessed the lowest waste generation, oppositely the large nations
131
owned the highest production. However, Italy had a relatively high amount of wastes
132
generation, while Bulgaria and Romania possessed relatively low level of waste.
133
It is reported that in urban and rural areas of EU and other Asia countries, the level
134
of solid wastes generation ranges from 0.9-1.6, and 0.7-1.5 kg per capita per day,
135
respectively while the urban areas generate approximately 1.3 billion tons of solid wastes
136
in 2015. It is estimated that it will increase to 2.2 billion tons by 2025 (WBA, 2017).
137
Generally, the produced solid wastes are composed by agricultural and industrial wastes
138
including leather, wood, glass, paper, green waste biomass, household wastes, and
139
electronic wastes, such as computers, telephones, refrigerators, and televisions as well as
140
also construction, demolition wastes and medical wastes (Munawar et al., 2019). As
141
mentioned above, in most of the developing countries, the massive population growth and
142
the acceleration of urbanization as well as the continuous improvement of people’s living
143
standards have significantly resulted in increasing the amount of urban wastes. The
144
population all over the world was about 7.2 billion in 2013 and it will reach to 9.6 billion
145
by 2050 (Biswabandhu et al., 2019). The population in urban areas is prognosed to account
146
for 86% and 64% of the total population in developed and developing countries,
147
respectively (Silva et al., 2019). Meanwhile, the dramatic increase of urban population
148
will also result in infrastructural and strategic challenges as well as adverse land-use
149
changes. Waste to energy technologies could turn as wise choices to deal with these
150
circumstances and provide long-term and cost-effective solutions for the growing energy
151
requirements and sustainable waste management. It is estimated that only from the global
152
urban waste collection market can produce around $410 billion income (Waste
153
Management World, 2017). However currently, only a quarter of wastes are recycled.
6
154
Hence, recycling and utilizing OSW by applying biotechnologies has great bright
155
prospects and market in the near future (Kiros et al., 2017).
156
3. Current Organic Solid Waste Treatment Technologies
157
Various options are available for converting organic solid wastes into useful
158
resources. Although pure cultures are ideal or synthesizing chemicals at high specificity
159
and thus requiring minimal purification prior to final use, they require single substrates in
160
the form of e.g. sugars. This has major drawbacks as it raises the cost of production and
161
also competes for resources required for growing food and feed for humans and animals
162
raising ethical debates. Natural mixed bacterial consortia are therefore an attractive option
163
for treating the abundantly available heterogeneous solid wastes and enables resource
164
recovery in the form of energy and high-value platform chemicals through anaerobic
165
digestion. These bacterial systems are also robust and have the capacity to withstand
166
stressful micro-environments. Among the biological solid waste treatment possibilities
167
provided in this section, only the production of biogas has been widely commercialized.
168
The other products of ADs are mainly at research stage, providing opportunities for further
169
scientific investigations and development (Wainaina et al., 2019a).
170
3.1. Biogas production
171
AD is an established biological processing technique suitable for stabilizing a
172
plethora of organic solid wastes coupled with recovery of energy and nutrients. It provides
173
a sustainable option for providing renewable materials and fuels thereby contributing to
174
circular economy (Antoniou et al., 2019). Indeed, there are indications of that the biogas
175
industry is steadily growing due to apparent substantial reduction of the capital and
176
operating costs of AD facilities. Indeed, it has been estimated that the cost of production
177
of biogas in AD facilities would reduce by about 38% in 2050 compared to 2015 (Ecofys,
178
2018). Globally, it the estimated that biogas production had an average growth rate of
179
11.2% reaching up to 58.7 billion Nm3 in 2017 (WBA, 2017).
7
180
The AD process takes place in oxygen-deficient environments and is catalyzed by
181
natural microbial communities in a four-stage complex process resulting into mainly
182
biogas. A pretreatment step, carried out either biologically, chemically or physically, can
183
precede the actual anaerobic digestion process in order to adequately prepare substrates
184
with difficult structures such as lignocelluloses (Taherzadeh and Karimi, 2008; Sarsaiya
185
et al., 2019a). The AD process kinetics are determined by the nature of substrates and the
186
physico-chemical parameters, such as the pH, temperature and hydraulic retention time
187
(Weiland, 2010). The details of the reactions taking place during the breakdown of the
188
solid wastes during AD are provided in previous reviews (Aryal et al., 2018; Merlin
189
Christy et al., 2014; Weiland, 2010). In summary, in the first stage hydrolytic microbes
190
break down the complex macromolecules into simple monomers with the help of enzymes
191
such as amylases, lipases, and proteases. The second stage applies acidogenic microbes
192
and converts the soluble products produced in the first step into biomolecules, such as
193
alcohols, volatile fatty acids (VFAs), hydrogen (H2) and carbon dioxide (CO2). These
194
products then serve as substrates to acidogens for synthesis of mainly acetic acid in the
195
third stage. The final stage is the most sensitive and greatly relies on the success of the
196
previous reactions in which methanogens convert acetic acid, hydrogen and carbon
197
dioxide into biogas. The methanogens follow mainly the acetotrophic (aceticlastic) and
198
Wood Ljungdahl pathways for production of CH4 from acetic acid and the reaction
199
between H2 and CO2, respectively, with the latter being more common (Aryal et al., 2018;
200
Merlin Christy et al., 2014; Rusmanis et al., 2019).
201
The chemical composition of raw biogas from a typical solid waste AD facility is
202
based on the process conditions. It consists of 50 – 75% CH4, 30 – 50% CO2, 0 – 3% N2,
203
~ 6% H20, 0 – 1% O2, 72 – 7200 ppm H2S, 72 – 144 ppm NH3 and other minor impurities
204
(Kapoor et al., 2019; Yentekakis and Goula, 2017). The raw biogas can be directly applied
205
for generating electricity and heat while upgraded gas (biomethane) can be injected into
206
the natural gas grid or used as vehicle fuel (Figure 1). The latter application has been
207
reported to having the potential of aiding in reducing road transport - related greenhouse
8
208
gas (GHG) emissions by up to 25% (Olsson and Fallde, 2015). In order to use the biogas
209
as a transport fuel, an extra treatment is required for removal of CO2 and other impurities,
210
such as H2S and water vapor. Gas cleaning is the initial step for removal of impurities that
211
could damage mechanical and electrical appliances during the use of biogas and can be
212
achieved using adsorption with silica gel and activated carbon or molecular sieves.
213
Upgrading techniques mainly for separation of CO2 from CH4 in order to raise the calorific
214
value of the gas are water scrubbing, pressure swing adsorption, cryogenic technology,
215
membrane separation and organic scrubbing using amines such as diethanolamine,
216
diglycolamine and monoethanolamine (Kapoor et al., 2019).
217
3.2. Hydrogen production
218
Energy recovery from solid wastes can also be achieved via the dark fermentation
219
process. Compared to the conventional production routes such as steam reforming of
220
methane and non-catalytic partial oxidation of fossil fuels, production of hydrogen from
221
wastes not only reduces the reliance on the depleting petroleum resources but at the same
222
time it is less energy-intensive as well (Kapdan and Kargi, 2006). Hydrogen has a higher
223
heating value compared to methane (120 vs. 50 MJ/kg) (Liu et al., 2013). It’s thus a
224
potential clean energy source especially for the transport industry. Its other areas of
225
application include production of electronics, processing of steel as well as hydrogenation
226
of fats and oils (Kapdan and Kargi, 2006). A number of organic solid wastes that have
227
been explored for hydrogen production are provided in Table 2.
228
The dark fermentation process is principally an intermediate within the AD
229
process in which hydrogen is released. Thus, relevant parameters suitable for biosynthesis
230
of hydrogen while at the same time deactivating the methanogens that would otherwise
231
consume it are optimized (Kapdan and Kargi, 2006). Some of the important process
232
parameters for ideal hydrogen production include the pH level, temperature and bioreactor
233
design. Fang and Liu (2002) reported an optimal pH level of 5.5 while mesophilic and
234
thermophilic temperature ranges were found to stimulate hydrogen production from
235
wastes compared to psychrophilic conditions (Wang and Wan, 2008; Wang and Wan,
9
236
2009). Hydrogen fermentation systems also require appropriate bioreactor design to
237
supply necessary hydrodynamic properties such as sufficient mixing (Ren et al., 2011). It
238
is worth mentioning that the presence of other by-products, such as volatile fatty acids
239
(VFAs) at certain concentrations can negatively impact on the microbial hydrogen
240
production due to potential inhibition or even the depletion of hydrogen associated with
241
formation of some VFAs. Potential solutions, such as the use of membrane bioreactors,
242
have been proposed as effective in removing the VFAs as they are being formed and
243
thereby improving the gas production (Trad et al., 2015).
244
3.3. Production of soluble biochemicals
245
3.3.1. Volatile fatty acids
246
VFAs are short-chain aliphatic carboxylic acids also formed as intermediaries
247
during the AD process before the methanogenesis stage and consist between 2 – 6 carbon
248
atoms. Petroleum resources are currently utilized for industrial synthesis of VFAs and
249
replacing these resources with solid wastes as the primary feedstock creates and attractive
250
circular economy pathway. The success of the acidification in AD reactors depends on
251
optimization of operating parameters, such the pH, temperature, organic loading rate
252
(OLR) and the hydraulic retention time (HRT) as well as employing appropriate means to
253
deactivate the methanogenic VFAs-consumers (Atasoy et al., 2018; Lim et al., 2008,
254
Wainaina et al., 2019a). Table 3 gives the performance of some waste-to-VFAs systems.
255
These acids have the potential for use as building blocks in a wide spectrum of
256
industrial processes. For instance, acetic acid can be applied in pharmaceutical industries,
257
propionic acid for manufacture of paints, butyric acid for manufacture of perfumes while
258
caproic acid is used for preparation of food additives. The mixed solution of these VFAs
259
also has important applications in wastewater treatment plants for biological removal of
260
nitrogen and phosphorus, biosynthesis of mixed alcohols as well as production of
261
biodegradable plastics. Moreover, the VFAs can be applied to generate electricity in
10
262
microbial fuels cells in addition to serving as the carbon source for bioprocessing of
263
biodiesel (González-García et al., 2019).
264
In order to facilitate practical production systems, recovery and purification of
265
VFAs from fermentation broths are crucial. This involves primary separation of the liquid
266
VFA fraction from the solid contents in the bioreactor made up of the cells and nutrients,
267
which might be sufficient for some applications. However, for some application areas,
268
more complex fractionation is needed for attaining the strict purity requirements.
269
Adsorption, gas stripping and electrodialysis are examples of relevant methods proved to
270
be effective in separation of the VFAs in model carboxylate solutions (Atasoy et al., 2018).
271
Some researchers have also designed systems with the recovery procedure running
272
concurrently with the actual microbial process in the so-called in situ product recovery
273
using membrane-aided techniques (Arslan et al., 2017; Wainaina et al., 2019b). The
274
benefits of such schemes are the supply of the carboxylates for subsequent processing
275
without introduction of foreign soluble chemicals, in addition to the relief on the microbes
276
from potential harmful effects of un-dissociated acid molecules.
277
3.3.2. Lactic acid
278
Lactic acid is a carboxylate with a market potential of USD 3.82 billion by 2020
279
used for production of acrylic acid, biodegradable polymers, pyruvic acid, 1, 2-
280
propanediol, among others (Bonk et al., 2017; Phanthumchinda et al., 2018). It’s main
281
area of application is as a building block for poly (lactic acid) which has a high estimated
282
market value of USD 5.16 billion by 2020 due to the mounting ecological concerns and
283
government policies (Phanthumchinda et al., 2018). A significant portion of the global
284
industrial production is from fermentation, which is controlled by factors such as pH,
285
temperature and nitrogen concentration (Alves de Oliveira et al., 2018).
286
The lactic acid bacteria (LAB) are popular for processing single sugars into lactic
287
acid. In the AD systems, a variety of organic residues, such as food wastes and organic
288
fraction of municipal solid waste, can be processed using microbes such as Lactobacilli
11
289
during the acidogenic phase (Gu et al., 2018; Probst et al., 2015). Gu et al. (2018) for
290
instance achieved a lactic acid buildup of up to 74.3% in a leach bed reactor treating food
291
waste. Another research by Bonk et al. (2017) developed an AD reactor coupled with a
292
percolation system which yielded lactic acid at 0.15 g chemical oxygen demand equivalent
293
per g total solids fed (gCOD/gTSfed) from food waste with lactic acid concentration of up
294
to 16.4 g COD/L. As with the VFAs, appropriate product separation and purification
295
methods are required to enhance the purity. One of the possible efforts involved a
296
membrane-assisted lactic acid recovery method that incorporated microfiltration for cell
297
removal, ultrafiltration for protein removal, and reverse osmosis for concentrating the
298
lactic acid at high purity (Phanthumchinda et al., 2018).
299
3.3.3. Other potential biochemicals
300
The AD process also synthesizes other useful metabolites, although in low
301
concentrations. According to Zhou et al. (2018), the possible metabolic pathways during
302
the acidogenic biodegradation of wastes can lead to biosynthesis of other chemicals
303
alongside VFAs, lactic acid and hydrogen. One such metabolite is succinic acid which can
304
be applied in key industrial processes such as manufacture of inks, polymers and
305
pharmaceuticals, and is bio-synthesized via the tricarboxylic acid cycle (TCA) (Isar et al.,
306
2006). The use of pure microbes such as Actinobacillus succinogenes and an engineered
307
Escherichia coli strain was found to effectively convert food waste hydrolysate into
308
succinic acid (Sun et al., 2014). Concentrations of up to 8.5 g/L were detected during
309
biodegradation of food waste at pH 5.0 (Lim et al., 2008). Such a performance indicates
310
the potential for mixed bacterial consortium for metabolizing succinate that could be
311
investigated in future studies. Ethanol has also been detected in small amounts from
312
anaerobic digestion digesters. For instance, acidification of food waste yielded low
313
concentrations at pH 6 and 7 (Xiong et al., 2019). In another study, Zhang et al. (2015)
314
found that ethanol made up 33.8% of the liquid fraction during the thermophilic AD of
315
glucose. One proposal to raise the concentration of ethanol by Zhou et al. (2018) would
316
be providing high H2 partial pressure in the head space of acidification reactors.
12
317
3.4. Nutrient recovery
318
The anaerobic digestion (AD) is considered as one of the foremost cost-effective
319
biological treatments of the biowaste. It permits vitality recuperation and the generation
320
of nutrient-rich digestate whereas lessening natural impacts of the waste transfer. It
321
contains most of the supplements from the input material and can be utilized as fertilizer
322
or organic amendment for farming. The natural supplement circle locally the digestate that
323
needs to be utilized in ranches found adjacent to the mAD (micro-anaerobic digestion)
324
units (Thiriet et al., 2020). The biowaste is utilized to provide vitality and recover other
325
nutrients not only because it advances towards the financial conditions of a country, but
326
besides makes a difference towards financial change of human society (Bhatia et al., 2018).
327
Recognizably, the two forms, i.e., anaerobic digestion and composting, facilitates together
328
since the solid fraction of digestate can be treated vigorously so as to recover both
329
renewable energy and nutrients from common waste (Micolucci et al., 2016). These
330
profitable nutrients display in natural strong waste which are efficiently misplaced, can be
331
recovered through appropriate biochemical technologies and utilized as nutrient-rich
332
fertilizers in rural areas for keeping up soil richness and arrive rebuilding hones at low-
333
input premise. In this way, impressive emphasis is being focused on receiving biologically
334
sound on best of financially maintainable and socially acceptable tools which reinforce
335
potential recuperation of nutrients, with least contamination load (Soobhany, 2019). The
336
final product, compost, may be a result of microbial action and contains mineral nutrients
337
such as nitrogen, phosphorus and potassium (Figure 6). Unfortunately, for nutrients
338
recuperation it is exceptionally weak material in its raw form, but can contribute in 9.4%
339
of P and 5% of N presented with inorganic fertilizers individually (Chojnacka et al., 2019).
340
TeraxTM technology gives numerous opportunities for resource recovery such as
341
changing over waste into value-added items, nutrient recuperation from natural waste, and
342
metal expulsion from natural waste (Munir et al., 2018). There are still various challenge's
343
heads for AnMBR stage applications to be handled, especially for extreme film fouling,
344
moo methane substance in biogas, exceedingly broken-down methane, destitute smelling
13
345
salts expulsion and phosphorus recuperation, etc. To address the above issues, a new-
346
generation process (Figure 6), i.e. so-called “Integrated Multistage Bio-Process (IMBP)”
347
constituted of solar-driven bioelectrochemical framework (BES)-AnMBR, fractional
348
nitritation/anammox (PN/A), nitrate decrease through anaerobic oxidation of methane
349
(AOM) and biological/chemical phosphorus precipitation units have been examined in
350
this article, with flexible capabilities in synchronous biowastes valorization, CO2 electro-
351
methanogenesis and synchronous biogas overhauling, in-situ fouling control, alkali
352
expulsion, broken up methane reutilization, and phosphorus recoup as hydroxyapatite-rich
353
supplements (Zhen et al., 2019).
354
3.5. Composting/co-composting
355
Composting process occurs naturally in the environment, although efficient
356
composting is needed to control several factors to avoid environmental challenges such as
357
odor and dust, and for obtaining a quality agricultural manure. Generally, composting
358
process are divided into two phases, the bio-oxidative phase and the maturing phase also
359
named as curing phase (Bernal et al., 1996). The bio-oxidative phase again carried out in
360
three stages, such as initial activation, thermophilic and mesophilic or maturation phase.
361
The organic waste degradation occurs during the thermophilic phase and microorganisms
362
degrade the available compounds in the organic waste (Keener et al., 2000). In this phase,
363
high temperature generation in the composting pile observed due to the heat generated
364
from the composting pile due to the microbial degradation of organic waste (Sarsaiya et
365
al., 2018). Generally, high temperature is required i.e. 55 oC to destroy pathogens in the
366
organic solid waste and usually composting piles reach temperatures as high as 70 oC
367
during the degradation of animal manure and green waste (Singh and Kalamdhad, 2014;
368
Tang et al., 2011; Cáceres et al., 2015). In addition, in this stage the organic compounds
369
degraded to CO2 and NH3 (Jeong et al., 2017; Kim et al., 2017), with the uptake of O2.
370
Finally, when the exhaustion of decomposable organic fraction in the waste achieved, the
371
temperature starts decreasing in the composting pile and hence beginning of the
372
maturation phase observed and stabilization and humification of the organic matter occur,
14
373
producing a mature compost with humic characteristics (Ravindran and Sekaran, 2010).
374
Hence, this compost can be characterized as the stable and sanitized product that make the
375
composting for sustainable agriculture and resource management (Haug, 1993; Ravindran
376
et al., 2019; Gajalakshmi and Abbasi, 2008).
377
Over the last decades, researchers have been focusing on the study of the complex
378
interaction between physical, chemical and biological factors that takes place during
379
natural composting under controlled conditions. There are several factors affecting the
380
composting process, such as nutrient balance, pH, porosity and moisture of the
381
composting mixture, aeration, moisture content and temperature. Thus, the control of
382
these parameters such as, porosity, nutrient content, C/N ratio, temperature, pH, moisture
383
and oxygen supply have established to be key factors for optimization of composting
384
process as they determine the optimal conditions for organic matter degradation and
385
microbial growth (Agnew and Leonard, 2003; Das and Keener, 1996; Haug, 1993;
386
Richard et al., 2002). The effectiveness of compost material with regard to their beneficial
387
effects on agricultural as a nutrient source, depends on the quality of the compost. The
388
quality criteria for compost material are determined by their nutrient content, organic
389
matter stabilization, presence of heavy metals and toxic compounds.
390
The traditional composting process results in partial decomposition, which has
391
phytotoxic effects in the final product (Ravindran et al., 2014). In order to overcome the
392
conventional composting limitations, several researchers have employed an in-vessel
393
composting strategy for different types of waste such as fly ash with organic waste
394
(Mandpe et al., 2019); swine manure (Awasthi et al., 2019a; Yang et al., 2019; Ravindran
395
et al., 2019); municipal solid waste (Malinowski et al., 2019); sewage sludge (Jain et al.,
396
2019). An in-vessel composting system provides a controlled environment, which
397
eliminates some of the obstacles inherent in traditional composting systems for producing
398
nutrient rich matured manure. Despite that, environmental concerns related to composting
399
existed, and can be categorized into three aspects described below;
400
3.5.1. Residual pollutants from the contaminated feed stock’s
15
401
Due to the existing contaminants in the feedstock, the levels of residual pollutants
402
partly determined the feasibility of composting in field utilization. Such contaminants
403
include heavy metals, organic pollutants, and emerging contaminants. Heavy metals refer
404
to metallic chemical elements that had a higher density and 4.5 cm3. Due to the high
405
toxicity, long persistence, and bio-magnification traits, heavy metal accumulation in soils
406
have become a global issue (Yang et al., 2017). Feedstock for composting, such as animal
407
manure or sewage sludge, generally contain a certain amount of heavy metals, and these
408
metals would be concentrated in the compost with the degradation of an organic
409
compound, although the bio-availability is reduced (Li et al., 2012). For example, a
410
nationwide survey on heavy metal content in compost derived from animals manures
411
found the concentration of Zn, Cu, Cr, Ni, Pb, As, Co, Cd, and Hg ranged from 11.8-3692,
412
3.6-916, 0.7-6603, 0.7-73, 0.05-189, 0.4-72, 0.1-94, 0.01-8.7, and 0.01-1.9 mg kg-1,
413
respectively (Yang et al., 2017). There were even higher levels of heavy metals in other
414
feed stock’s like sewage sludge (Awasthi et al., 2017, 2018). Accordingly, government
415
across different countries have introduced strict standard to regulate the maximum
416
permissible values for organic compost.
417
Organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) such as
418
polychlorinated biphenyl (PCB), pesticides, and petroleum were generally found in
419
compost (Ren et al., 2018; Houot et al., 2012). Even though the concentrations of such
420
pollutants were generally reduced through composting via biodegradation, the
421
concentration of residual organic pollutants might be higher than the background levels,
422
which means inputs of such compost should be limited (Brandli et al., 2007). The other
423
residues pollutants are antibiotic resistance genes and emerging contaminants such as
424
micro particle, hormone, and perfluorinated compounds (PFCs). There were also high
425
content of antibiotic in the common feedstock’s of composting (i.e., sludge and manures),
426
and studies focused on the fate of antibiotics and antibiotics resistance genes have become
427
a research hotspot since 2009 (Ezzariai et al., 2018). However, studies on the
428
transformation of antibiotic resistance genes are still limited, and the obtained results are
16
429
partly contracted with each other (Wu et al., 2011; Wang et al., 2015). Therefore, more
430
detailed research on the fate of antibiotic residence genes during composting should be
431
further investigated.
432
3.5.2. Odors
433
During composting, series of stinky gases such as hydrogen sulfide, volatile organic
434
sulfides, ammonia, pyridine, amines, hydrocarbons, terpenes, alcohols, ketones,
435
aldehydes, and esters were generated (Wei et al., 2017). Additionally, the constitution and
436
concentration of different compounds varied significantly and affected by the feed stock
437
properties, composting operation conditions, and the ambient environment (Awasthi et al.,
438
2018). Even though their concentrations are quite low, emission of odors gases can lead
439
to significant allergy among the neighbor. Fortunately, we can significantly reduce the
440
emission of odors nowadays by mediating ingredient properties, enhancing aeration,
441
providing catalysts and inoculant, and building airtight composting facilities. More
442
specifically, providing enzymatic catalysts on the surface of a compost pile or in the
443
airspace above it can facilitate the biological reaction and eventually degrade the odorous
444
compounds. Additionally, the addition of straw materials, chemical reagents (such as
445
FeCl3, lime, and MgO), and biochars can effectively prevent the emission of odors during
446
composting (Ren et al., 2018; González et al., 2019).
447
4. Challenges and prospects of bio-based circular economy
448
The term “bio-based circular economy” can refer to an approach that promotes the
449
reuse and valorization of organic wastes and residues. In the recent years, it has more
450
essential options for the conversion of bio-waste and their intermediate products into
451
bioenergy or secondary raw materials (Mohan et al., 2016).
452
4.1. Integrated organic solid waste treatments technologies
453
Rapid urbanization is surely creating enormous pressure on the urban bodies to
454
deal with the demands of the developing populace and the consistently expanding huge
455
amounts of waste all through the world (Awasthi et al., 2017; Zhou et al., 2014; Zhao et
17
456
al., 2014). Because of lack of information and inappropriate waste gathering frameworks
457
the untreated waste eventually arrives at the landfills and are responsible of aggravating
458
the natural conditions. Inability in completely grasping all the issues identified with the
459
management of municipal solid waste (MSW) ends up being one of the most challenging
460
issues of degrading the environment. MSW contains a significant portion (30–50%) of
461
organics (Bhattacharyya et al., 2008). The amount of each waste stream within the total
462
solid waste generated varies with changes in specific parameters (Arafat et al., 2013).
463
At present, waste is viewed as an important asset rather than recognizing it to be
464
useless and resource extraction from them is one of the primary goals of sustainable waste
465
management (Kumar, 2017). A few treatment technologies are feasible with varying waste
466
management abilities (Arafat et al., 2013). However, no single technique can fix the issue
467
of waste management (Tehrani et al., 2009). Therefore, an integrated approach towards
468
the treatment of solid waste is of great importance. It not just gives an adaptable waste
469
treatment decision (organic or inorganic) but energy and asset recovery can likewise be
470
effectively accomplished through it (Zhou et al., 2014). The significant reason behind
471
treating the organic waste is to recover recyclables and energy from it and furthermore to
472
improve the quality of the waste handling it (Figures 4 and 5).
473
Organic solid waste treatment technologies may incorporate either a combination
474
of mechanical, thermal or biological treatments or any of the three separately. The
475
significant ones being the thermal and biological treatment techniques (Kumar, 2017). An
476
integrated approach is certainly required which should be from its generation to its final
477
disposal. The treatment procedure chosen ought to be such that it accomplishes the
478
objectives of waste management. When the waste is explicitly organic in nature the
479
biological procedures plays the main job, which are the aerobic and anaerobic digestion,
480
bio-landfilling, bio-drying and vermicomposting. Aerobic digestion transforms the easily
481
degradable organic waste aerobically into soil conditioners and carbon dioxide with
482
resulting heat production (Polprasert, 2007). Whereas anaerobic digestion involves
483
degrading the organic waste anaerobically which leads to recovery of energy in form of
18
484
biogas (Tchobanoglous et al., 2004). In this process, the organic waste gets decomposed
485
by different microbial actions. However, in comparison to aerobic digestion, anaerobic
486
digestion produces less solid sludge (Henze et al., 2008). Bio-landfilling is another
487
procedure utilized for managing the solid waste that upgrades the waste degradation and
488
also increases the measure of biogas formation (Davis and Cornwell, 2008). Anaerobic
489
processing and bio-landfilling varies just in the utilization of reactor type which is simply
490
the landfill itself during bio-landfilling. Bio drying is another such approach that aides in
491
diminishing the measure of water present in the natural waste which would some way or
492
another hamper the energy recuperation process (Polprasert, 2007). It additionally helps
493
in saving energy for usage in residue derived fuels (Adani et al., 2002). All the major
494
integrated organic waste treatment technologies discussed above have been shown in
495
Figure 5. All these technologies require a legitimate identification and assessment of
496
accessible options, planning, collection, isolation, treatment and final disposal. In addition,
497
understanding the potential interrelationships between various treatment options would
498
help in building up an integrated approach where all the individual techniques supplement
499
one another.
500
4.2. Techno-economic feasibility of aerobic and anaerobic digestion technologies
501
“Wastes from one industrial process can serve as the raw materials for another,
502
thereby reducing the impact of industry in the environment” was described by Frosch and
503
Gallopoulos (1989). Eco-industrial parks are making collaborate with other inter-company
504
to optimize the resource efficiency. They can share the resources, water, and energy.
505
Whereas working together, the community of businesses looking for collective benefits
506
rather than individual benefits. Hence, the eco-industrial parks make industrial symbiosis
507
for improvement of their economic status by sharing the raw materials and wastes. An
508
anaerobic digestion process is a process, that can biochemically modify the whole biomass
509
into specific components and this process can produce carbon dioxide, phosphorus and
510
ammonia to improve the economic and environmental benefits. The biochemical
511
processes in anaerobic digestion are nature. It is accepted and established as an innovative
19
512
technology to produce the bio energy from whole biomass (Kavitha et al., 2017; Stiles et
513
al., 2018).
514
An assessment of techno-economic analysis and life cycle are essential for any
515
processes to be sustainable, which can be analysis by three parameters like economical
516
viability, technical feasibility and environmental sustainability. There are number of
517
industries and investors looking the production of biogas using anaerobic digestion
518
method for generation of energy and a high-risk investment, because of its process like
519
feedstock supply, segregation of solid waste, characteristics features of feedstock, profit
520
issues and optional problems (Schmidt, 2014). Based on the above facts, the many
521
governments have support and encourage by provide the funds to company for this area.
522
According to Waste Management World (2017), the global biogas market will reach $50
523
billion on 2026, hence it is crucial for scientist and researchers to conduct their research
524
on anaerobic digestion at systems level, as well as techno-economic and environmental
525
benefits perspective. So, it is very essential to assess integrated analyses of economic
526
viability and technical feasibility on anaerobic digestion technologies. The economic
527
analysis for particular process is analyzed by values of materials for construction, interest
528
rate and labor cost (Murthy, 2016). The techno-economic analysis and life cycle
529
assessment (LCA) are providing the insights on the feasibility of a process and it is very
530
important for bioenergy and bio-based products (Murthy, 2016). Techno-economic
531
evaluation determines that maximum funds spent in biogas purification for various
532
applications including the power-to-gas, electricity and vehicle fuel. Various anaerobic
533
digester upgrading methods are available with membrane separation, water scrubbing,
534
pressure adsorption and solvent scrubbing. Hence, the government has high cost for
535
upgrading the anaerobic systems (Bauer et al., 2013; International Energy Agency, 2015).
536
Anaerobic co-digestion process is gradually more used to change the liquid and
537
solid waste into nutrient rich content and reduce the impact of noxious compounds in the
538
process and enhance the yield of biogas (Carlini et al., 2017). It is considered as the most
539
important method for the management of biomass to production of biogas, and the
20
540
digestate can be used as bio-fertilizer. This process can completely eliminate the
541
pathogens (Sarsaiya et al., 2019b; Sarsaiya et al., 2019c) and to prevent the emission of
542
greenhouse gases (Torquati et al., 2014; Weiland, 2010). United Kingdome and Hong
543
Kong have strong-willed to sponsor the use anaerobic digestion methods for energy
544
production and recovery. It also fulfils the Sustainable Development Goals (SDGs) for
545
development of renewable energy (HK Policy address, 2017). In Italy, there are 35% of
546
biogas plants that produce 70 – 100 MW electricity, which straight relate to government
547
incentives (Gestore Servizi Energetici, 2015).
548
During the techno-economic analysis, three areas of anaerobic digestion process
549
are important: (1) unit operations with collection and transport of feed stocks, (2) to
550
provide the treatment facility for production of biogas, and (3) upgrading the bio-energy
551
for various applications as electricity and liquid methane for household cooking. There is
552
20-50% of budget allotted to collect and transport the waste by manual collection or
553
trucks/trailers in the sector of municipalities and corporations (Daniel and Perinaz, 2012).
554
In Ireland, cost of $22 – 35/month was spent to waste collection from various sectors like
555
houses and industries for anaerobic digestion (Koushki et al., 2004). Figure 2 shows the
556
cost associated with complete value chain of anaerobic digestion for solid waste.
557
4.3. Proposed models for a circular economy
558
4.3.1. Co-digestion
559
Biodegradation of more than one type of waste in one anaerobic bioreactor, albeit
560
in appropriate proportions, is a promising technique for increasing product yields ascribed
561
to the balance of nutrients that enhances microbial performance coupled with the
562
economic benefits of treating a variety of materials in a single facility (Abad et al., 2019;
563
Awasthi et al., 2019b; PagésDíaz et al., 2011). Several researchers have observed
564
improvement of AD systems from a number of solid wastes demonstrating that anaerobic
565
co-digestion, illustrated in Figure 3a, could be a favorable method for achieving circular
566
economy.
21
567
A study by Panichnumsin et al. (2010) reported that an increase of 41% in the
568
specific methane yield would be realized when a cassava pulp/pig manure mixture was
569
applied in the ratio of 3:2 relative to the use of pig manure alone. The potential cause of
570
the improved performance was the improved process stability as well as an increase in the
571
amounts of easily degradable macromolecules (Panichnumsin et al., 2010). In another
572
study, improved buffering capacity and suitable carbon-to-nitrogen ratio were found to
573
facilitate a stable AD process at high organic load even without regulation of pH when
574
food waste and cattle manure were in ratio of 2:1 leading to an increase in the methane
575
production by 55.2% compared to food waste alone (Zhang et al., 2013). The co-digestion
576
strategy is also beneficial when targeting other AD bio-products such as H2 and VFAs. A
577
research by Zhou et al. (2013b) examined several mixtures containing food waste, primary
578
sludge and waste activated sludge and concluded that co-digestion of the substrates
579
improved hydrogen production compared to single substrates. The optimal increase was
580
realized when a food waste/ primary sludge/waste activated sludge mixture was applied
581
at a ratio 80:15:5 (Zhou et al., 2013b). The impact of co-digesting waste activated sludge
582
and corn straw was investigated by Zhou et al. (2013a). They observed an improvement
583
in the maximum VFAs yield reaching up to 69% when a ratio of 1:1 of the mentioned
584
substances was used compared to sludge only in addition to the manipulation of the
585
product profile. Further results indicated that protein and carbohydrate consumption were
586
enhanced by more than 120 and 220%, respectively (Zhou et al., 2013a).
587
Besides the conventional wet digestion, solid wastes can also be biodegraded at
588
high total solids content of between 15 – 40% in the so-called dry AD processes using e.g.
589
horizontal plug flow, sequential batch, textile-based, and vertical plug flow reactors. Dry
590
AD is a promising technology for regions with low water availability and might also evade
591
the pretreatment step for recalcitrant materials in addition to requiring more compact
592
bioreactors (Awasthi et al., 2019a). An increase in methane yield of 14% was achieved
593
when citrus wastes, chicken feather and wheat straw at a ratio of 1:1:6 in dry batch AD
594
essays compared to single substrates. When the same substrate ratio was used in a
22
595
continous plug flow bioreactor, a yield of 362 NmLCH4/gVSadded at a loading rate of 2
596
gVS/L/d was obtained (Patinvoh et al., 2018). Another dry AD process was demonstrated
597
by Capson-Tojo et al. (2017) in which food waste and cardboard were co-digested in batch
598
mode. Their research focused on the impact inoculum-to-substrate ratio which was found
599
to have a significant influence on the methane yield, microbial dynamics as well as the
600
productions of hydrogen and VFAs.
601
4.3.2. Recovery of multiple products
602
The AD process can be employed for the recovery of more than a single metabolite
603
at reasonable yields, thereby increasing the process conversion efficiency (Figure 3b and
604
5). One of the possible scenarios was recently successfully executed involving the
605
production of both VFAs and biogas from urban biowastes (organic fraction of municipal
606
solid waste and waste activated sludge) in a pilot two-stage AD process (Valentino et al.,
607
2019). The acidification reactor in the first stage operated at 37 °C generated VFAs up to
608
at 19.5 g COD/L without external pH control. In the second stage, the solid-rich fraction
609
collected from the acidification reactor was converted into biogas containing 63–64% CH4
610
at a yield of up to 0.40 m3/kg VS at 37 °C. Furthermore, it was claimed that a similar
611
scaled-up system at the mentioned temperature would be thermally sustainable leading to
612
a surplus of thermal energy in addition to an estimated revenue stream of 609,605 €/year
613
from electricity (Valentino et al., 2019). Another scenario would involve the co-
614
production of lactic acid and methane. Using organic fraction of municipal solid waste,
615
Dreschke et al. (2015) designed a process to produce lactic acid in batch reactors at 37 °C.
616
Lactic acid made up to 83% of the acid content with a yield of up to 37 g/kg was reached
617
after 24 hours. They also reported 75% Lactobacilli enrichment in the microbial
618
consortium. The effluent from the batch fermenters were then co-digested with digested
619
municipal sludge and a methane yield of up to 618 NmL CH4/gVS was attained. In
620
addition, the final effluent was proposed as a potential source of organic fertilizer
621
(Dreschke et al., 2015).
23
622
In addition to the production of dissolved metabolites, it has been demonstrated
623
the AD process can be applied for biosynthesis of both hydrogen and methane. The system
624
can be optimized to yield the separate products in two distinct stages to produce a mixture
625
known as hythane, which usually contains 10–25% hydrogen in methane (Liu et al., 2013;
626
Wang et al., 2011). In addition to the possibility of effectively optimizing the separate
627
processes, the other advantages of hythane production, relative to the single hydrogen or
628
methane production, include the reduced overall fermentation time and the enhanced
629
energy recovery (Liu et al., 2013). For instance, Siddiqui et al. (2011) observed that a
630
single phase AD reactor resulted in 0.48 m3/kgVSdestroyed methane yield, while a two-phase
631
process produced 0.67 m3/kgVS destroyed methane in addition to hydrogen at a yield of
632
0.13 m3/kgVS destroyed. Elsewhere when pulp and paper sludge and food waste (on a 1:1
633
mixing ratio based on the VS) were used, Lin et al. (2013) demonstrated a two-stage
634
process in which hydrogen was produced in mesophilic conditions in stage one while the
635
digestate from this stage was subjected to thermophilic AD conversion. They reported no
636
accumulation of VFAs in the first reactor that yielded hydrogen up to 64.48 mL/gVSfed at
637
pH levels 4.8–6.4 while the methane yield was 432.3 mL/gVSfed at pH levels 6.5–8.8 in
638
the second reactor. In another research, Wang et al. (2011) co-digested cassava stillage
639
and excess sludge in thermophilic conditions and found a 25% higher energy yield when
640
a two-phase fermentation system was carried instead of a one phase system. By mixing
641
cassava stillage and excess sludge at ratio of 3:1 (on VS basis) a hydrogen yield of 74
642
mL/gVSfed and a methane yield of 350 mL/gVSfed were attained.
643
4.3.3. Combined bioprocesses
644
Another strategy to implement systems that potentially meets the circular economy
645
criteria involves combination of processes that are independently optimized to recover
646
renewable energy, chemicals and materials (Figure 3c). Successful demonstrations of such
647
schemes mainly comprise established industrial processes involving the production of
648
ethanol from lignocellulosic materials as the major activity. For instance, wheat straw was
649
converted into three important products: ethanol, hydrogen and biogas (Kaparaju et al.,
24
650
2009). The integrated bioprocess began with hydrothermal pretreatment of the wheat
651
straw which resulted into cellulose-rich solid fraction and a hemicellulose-rich liquid
652
fraction. The solid fraction was applied for ethanol fermentation in which a yield of up to
653
0.41 g/g-glucose was realized. Biogas was then produced using the effluent from the
654
ethanol production processes yielding up to 0.324 m3 CH4/kg VSadded. The liquid fraction
655
from the straw pretreatment was separately used in dark fermentation reactors and yielded
656
up to 178.0 mL H2/g-sugars. Similarly, the effluent from the hydrogen production
657
processes was anaerobically digested to produce biogas at a yield of up to 0.381 m3
658
CH4/kg VSadded. Woody biomass has also been applied as a feedstock for production of
659
both ethanol and biogas. Safari et al. (2016) during an investigation on the performance
660
of dilute H2SO4 pretreatment of softwood pine demonstrated the potential improvements
661
in the ethanol and biogas productions. Similar to the previously described work, the solid
662
fraction from the pretreatment, consisting mostly of cellulose, was applied to produce
663
ethanol production while the liquid fraction consisting of sugars was applied to produce
664
biogas. The authors reported the highest ethanol yield of 53% while the highest biogas
665
production yield was 162 m3 methane/ton of pinewood. Another key outcome from their
666
experiments showed that the inhibitory compounds released at the pretreatments at 180
667
°C had more negative impact on the production of biogas compared to the production of
668
ethanol (Safari et al., 2016).
669
Another study was made by Wachendorf et al. (2009) proposed the use of silage
670
for production of biogas and processing of a solid fuel as a potential feed stock for
671
combustion or gasification. The silage was first subjected to hydrothermal conditioning
672
and then mechanically dehydrated to separate the liquid from the solid portions. The
673
former portion was used as substrate for production of biogas while the latter portion
674
remained as a press cake (solid fuel). Moreover, the digestate from the anaerobic digester
675
could be used as fertilizer for growing more silage, making the proposed scheme highly
676
efficient in resource recovery (Wachendorf et al., 2009). In addition, a promising model
677
that could be appropriate for agricultural wastes would be the simultaneous recovery of
25
678
essential materials such as edible fungi. Nair et al. (2018) demonstrated an integrated
679
bioprocess in which biogas, ethanol and protein-rich fungal biomass from pretreated
680
wheat straw. Neurospora intermedia was the species applied for producing ethanol
681
(yielding up to 90% of the theoretical maximum) as well as the biomass. Further results
682
revealed that compared to untreated straw, an increase of up to 162% in the methane yield
683
would be realized from the anaerobic treatment of the liquid stream from the pretreatment
684
process. Besides, co-digestion essays of the mentioned stream with thin stillage, a waste
685
stream from the ethanol production process, resulted in an increase in the total energy
686
output of up to 27% (Nair et al., 2018).
687
5. Conclusion
688
This review paper evaluated resource recovery and circular economy options from
689
organic solid waste using aerobic and anaerobic digestion technologies. Utilization of
690
organic waste residues would elevate biorefinery concept and social acknowledgments.
691
Aerobic and anaerobic digestion are widely acceptable technologies to enhance the
692
efficiency for energy and other high value bio-based products production. Overall, various
693
advanced models have recently been proposed and already worked out in line with a
694
holistic biorefinery avenue that bring enormous industrial importance. Hence, these
695
technologies should be critically assessed as an interracial pyramid scheme and a techno-
696
economic feasibility examination of this plan is also needed.
697
Acknowledgments
698
This work was supported by a research fund from the International Young
699
Scientists from National Natural Science Foundation of China (Grant No. 31750110469),
700
China; the Shaanxi Introduced Talent Research Funding (A279021901), and The
701
Introduction of Talent Research Start-up Fund (Grant No. Z101021904), College of
702
Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi
703
Province 712100, China. The Mobility for Regional Excellence-2020 (Grant No. RUN
704
2017-00771), European Union's Horizon 2020 Research and Innovation Programme
26
705
under the Marie Skłodowska-Curie grant agreement No. 754412 at University of Borås,
706
Borås, Sweden, and Guizhou Science and Technology Corporation Platform Talents Fund,
707
China (Grant No.: [2017]5733-001 & CK-1130-002). We are also thankful to all our
708
laboratory colleagues and research staff members for their constructive advice and help.
709
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710
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1113
fractionated dissolved organic matter on electron transfer capacity during
1114
composting. Chinese J. of Anal. Chem. 45, 579-586.
1115
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Yang, W, Zhuo, Q., Chen, Q., Chen, Z., 2019. Effect of iron nanoparticles
1116
on passivation of cadmium in the pig manure aerobic composting process, Sci.
1117
Total Environ. 690, 900-910.
1118
138.
Yentekakis, I.V., Goula, G., 2017. Biogas management: Advanced
1119
utilization for production of renewable energy and added-value chemicals. Front.
1120
Env. Sci. 5.
1121 1122 1123
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Zhang, C., Xiao, G., Peng, L., Su, H., Tan, T., 2013. The anaerobic co-
digestion of food waste and cattle manure. Bioresour. Technol. 129, 170-176. 140.
Zhang, F., Chen, Y., Dai, K., Shen, N., Zeng, R.J., 2015. The glucose
1124
metabolic distribution in thermophilic (55°C) mixed culture fermentation: A
1125
chemostat study. Int. J. Hydrogen Energ. 40, 919-926.
1126
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Zhang, L., Kai-Chee, L., Zhang, J.X., 2019. Enhanced biogas production
1127
from anaerobic digestion of solid organic wastes: Current status and prospects.
1128
Bioresour. Technol. Reports. 5, 280-296.
41
1129
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Zhao, P., Shen, Y., Ge, S., Chen, Z., Yoshikawa, K., 2014. Clean solid
1130
biofuel production from high moisture content waste biomass employing
1131
hydrothermal treatment. Appl. Energ. 131, 345-367.
1132
143.
Zhen, G., Pan, Y., Lu, X., Li, Y.Y., Zhang, Z., Niu, C., Kumar, G.,
1133
Kobayashi, T., Zhao, Y., Xu, K., 2019. Anaerobic membrane bioreactor towards
1134
biowaste biorefinery and chemical energy harvest: Recent progress, membrane
1135
fouling and future perspectives. Renew. Sust. Energ. Rev. 115, 109392.
1136
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Zhou, A., Guo, Z., Yang, C., Kong, F., Liu, W., Wang, A., 2013a. Volatile
1137
fatty acids productivity by anaerobic co-digesting waste activated sludge and corn
1138
straw: Effect of feedstock proportion. J. Biotechnol. 168, 234-239.
1139
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Zhou, H., Meng, A., Long, Y., Li, Q., Zhang, Y., 2014. An overview of
1140
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1141
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1143
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1144
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1146
hydrogen production for anaerobic co-digestion of food waste and waste water
1147
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1148
Figure captions:
1149
Figure 1. Potential feed stock, conversion technologies and products
1150
Figure 2. Biogas production and the potential applications.
1151
Figure 3. Proposed models for circular economy using anaerobic digestion technology:
1152
co-digestion using municipal, industrial and agricultural wastes (a), recovery of multiple
1153
products (b) and integration of bioprocesses (c).
1154
Figure 4. Schematic presentation of integrated food waste and wastewater treatment
1155
method (MOWFAST) and conventional sludge (PSSS) treatment by anaerobic digestion
1156
(Extracted from Kaur et al., 2019)
42
1157
Figure 5. Various costs associated with the value chain of waste treatment from collection
1158
to products generation. (Extracted from Rajendran and Murthy, 2019).
1159
Figure 6. Different technologies and processes for the utilization of organic solid waste
1160
for nutrients recovery.
1161 1162 1163 1164 1165
1166 1167
Figure 1: Potential feed stock, conversion technologies and products
43
1168
1169 1170
Figure 2: Biogas production and the potential applications
44
1172 1173 1174 1175
Figure 3: Proposed models for circular economy using anaerobic digestion technology: co-digestion using municipal, industrial and agricultural wastes (a), recovery of multiple products (b) and integration of bioprocesses (c)
45
1176 1177
Figure 4: A Schematic presentation of integrated food waste and wastewater treatment
1178
method (MOWFAST) and conventional sludge (PSSS) treatment by anaerobic digestion
1179
(Extracted from Kaur et al., 2019)
46
1180 1181
Figure 5: Various costs associated with the value chain of waste treatment from collection
1182
to products generation (Extracted from Rajendran and Murthy, 2019)
47
1183 1184
Figure 6: Different technologies and processes for the utilization of organic solid waste
1185
for nutrients recovery
1186
148.
1187
Tables captions:
1188
Table 1. Wastes production by economic and household activities until 2016 on
1189
percentage basis.
48
1190
Table 2. Potential organic waste substrates and optimal conditions for hydrogen
1191
production.
1192
Table 3. Potential organic waste substrates and optimal conditions for production of
1193
volatile fatty acids.
1194 1195 1196 1197
49
1198
Table 1. Countries
Mining and quarrying
Manufacturing
Energy
Constru ction and demoliti on
Other economic activities
Households
EU-28 Belgium Bulgaria Czechia Denmark Germany Estonia Ireland Greece Spain France Croatia Italy Cyprus Latvia Lithuania Luxemburg Hungary Malta Netherland s Austria Poland Portugal Romania Slovenia Slovakia Finland Sweden United Kingdom Iceland Liechtenste in Norway
25 0 82 1 0 2 26 16 78 16 1 12 0 5 0 1 0 1 8 0
10 23 3 18 5 14 37 35 6 11 7 8 17 33 19 41 7 17 1 10
3 1 8 4 4 3 25 2 4 3 0 2 2 0 11 2 0 16 0 1
36 31 2 40 58 55 5 10 1 28 69 24 33 36 4 8 75 23 69 70
16 36 3 23 16 17 6 28 4 26 14 31 29 10 30 32 11 25 13 13
8 8 2 14 17 9 2 10 7 17 9 22 18 16 34 17 6 18 8 6
0 39 3 87 0 3 76 77 6
9 17 17 4 26 32 8 4 4
1 11 1 4 14 9 1 1 0
73 10 12 0 10 9 11 7 49
10 18 35 3 38 29 3 7 30
7 5 33 2 12 18 1 3 10
0 3
25 2
0 0
4 88
31 1
40 5
3
14
2
27
32
22
50
1199
Montenegr o North Macedonia Serbia Turkey Bosnia and Herzegovin a Kosovo
19
2
18
37
10
13
49
51
0
0
0
0
79 11 2
3
1
2
27
12 26 71
0
0
3 37 0
14
20
40
6
10
11
Source: Eurostat (online data code: env_wasgen)
1200 1201 1202
Table 2. Substrate
Operating conditions
Hydrogen production
Reference
performance Melon waste
Continuous fermentation
Yield= 352 mL/gVSadded
T= 55 °C
(Akinbomi and Taherzadeh, 2015)
HRT= 5 days pH= 5.0 Melon waste
Batch essay
Productivity= 351.12
T= 36 °C
mLH2/Lreactor.h
(Turhal et al., 2019)
pH= 5.5 – 6.0 Apple waste
Continuous fermentation
Yield= 635 mL/gVSadded
T= 55 °C
(Akinbomi and Taherzadeh, 2015)
HRT= 5 days pH= 5.0 Mixed food waste
Continuous fermentation
Yield= 1.80mol-H2/mol-
T= 55 °C
hexose
(Shin et al., 2004)
HRT= 5 days pH= 5.5 Mixed food waste
Batch essay
Yield= 176.10 mL
(Wongthanate and
T= 55 °C
H2/gCOD
Mongkarothai, 2018)
Initial pH= 7.0
51
Waste peach pulp
Batch essay
Productivity = 35.6
T= 37 °C
mLH2/h
(Argun and Dao, 2017)
Initial pH= 5.9 Fruit and vegetable waste
Batch essay
Yield= 76 mL H2/g VS
(Keskin et al., 2018)
Continuous fermentation
Production rate = 7.4
(Han et al., 2016)
T= 55 °C
LH2/L/d
T= 55 °C Initial pH= 7.0 Waste bread
HRT= 6 hours pH˃ 4.0
1203
52
1204
Table 3. Solid waste
Optimal conditions
Production performance
Reference
Olive mill solid waste
Batch-fed fermentation
Concentration=3.69
(Cabrera et al., 2019)
T= 35 °C
gCOD/L
pH= 9.0 Mixed food waste
Semi-continuous
Yield= 0.54 gVFA/ VSfed
(Wainaina et al., 2019a)
Yield = 0.39 gVFA/ VSfed
(Lim et al., 2008)
Yield = 0.46 gCOD/gCOD
(Garcia-Aguirre et al.,
fermentation * T= 37 °C HRT= 6.67 days OLR= 2 gVS/L/d pH= 3.6 – 4.1 Mixed food waste
Semi-continuous fermentation T= 35 °C HRT= 12 days pH= 5.5
Meat and bone meal
Batch fermentation T= 55 °C
2017)
pH= 10.0 Organic fraction of
Continuous fermentation
Yield = 0.31
municipal solid waste
T= 55 °C
gCOD/gCODin
HRT= 3.3 days OLR= 20.5
(Valentino et al., 2018)
VFA concentration=16.0
kgVS/m3/
d
gCOD/L
pH ≥ 4.0 Waste activated sludge
Batch fermentation
Concentration=2708.02
T= 21 °C
mg/L
pH= 10.0
1205
* A membrane bioreactor was used for simultaneous fermentation and product recovery
1206 1207 1208 1209
53
(Chen et al., 2007)
1210 1211 1212 1213
149.
1214
Highlights:
1215 1216
Reviewed resource recovery from organic solid waste towards bio-circular economy.
1217 1218
Aerobic and anaerobic digestion technology: a sustainable approach for biorefinery.
1219
Organic solid waste: a potential biorefinery and bio-economy option.
1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239
150. Steven Wainainab, Dr. Steven is write 60% part of this manuscript Mukesh Kumar Awasthia*,b, He has design and prepared the content of review and also write 30% part of this manuscript Surendra Sarsaiyaf, He has help to prepared the figures Hongyu Chend, He has help to write introduction and prepared table 1. Ekta Singhc, He has put some contribution to arranged and crossed check references Aman Kumarc, He has put some contribution to arranged and crossed check references B. Ravindrane, He has put some contribution to write nutrient recover section Sanjeev Kumar Awasthia, He has put some contribution to arranged and crossed check references Tao Liua,
54
1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252
He has put some contribution to arranged and crossed check references Yumin Duana, He has put some contribution to arranged and crossed check references Sunil Kumarc, Dr. Kumar is very help to improve the manuscript quality and English Zengqiang Zhanga, Prof. Zhang is very help to improve the manuscript quality Mohammad J. Taherzadehb Prof. Mohammad is very help to improve the manuscript quality and English
151. Declaration of interests
1253 1254 1255 1256
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
1257 1258 1259 1260
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
With warm regards,
Dr. Mukesh Kumar Awasthi
Associate Professor
1261 1262 1263
College of Natural Resources and Environment, Northwest A&F University, Taicheng Road 3#, Yangling, Shaanxi 712100, China.
55
1264 1265
152.
56